Thanks for trying to help.
Maybe it's the way the game is, i.e get a few large items in your park such a big pool complex with water slides, big coasters etc and it just slows down and is really jerky.
Does anyone else have this problem, or does it seem apparent that there's something amiss with my computer(s)?

It gets a little jerky on my PC also. So you're not alone. Here's what I have.
------------------
System Information
------------------
Time of this report: 6/20/2006, 22:31:05
       Machine name: BEAST
   Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 2 (2600.xpsp_sp2_gdr.050301-1519)
           Language: English (Regional Setting: English)
System Manufacturer: NVIDIA
       System Model: AWRDACPI
               BIOS: Phoenix - AwardBIOS v6.00PG
          Processor: AMD Athlon™ 64 X2 Dual Core Processor 4200+,  MMX,  3DNow (2 CPUs), ~2.2GHz
             Memory: 1024MB RAM
          Page File: 276MB used, 2184MB available
        Windows Dir: C:\WINDOWS
    DirectX Version: DirectX 9.0c (4.09.0000.0904)
DX Setup Parameters: Not found
     DxDiag Version: 5.03.2600.2180 32bit Unicode

------------
DxDiag Notes
------------
  DirectX Files Tab: No problems found.
      Display Tab 1: No problems found.
        Sound Tab 1: No problems found.
        Sound Tab 2: No problems found.
          Music Tab: No problems found.
          Input Tab: No problems found.
        Network Tab: No problems found.

--------------------
DirectX Debug Levels
--------------------
Direct3D:    0/4 (n/a)
DirectDraw:  0/4 (retail)
DirectInput: 0/5 (n/a)
DirectMusic: 0/5 (n/a)
DirectPlay:  0/9 (retail)
DirectSound: 0/5 (retail)
DirectShow:  0/6 (retail)

---------------
Display Devices
---------------
        Card name: NVIDIA GeForce 6600
     Manufacturer: NVIDIA
        Chip type: GeForce 6600
         DAC type: Integrated RAMDAC
       Device Key: Enum\PCI\VEN_10DE&DEV_0141&SUBSYS_C3693842&REV_A2
   Display Memory: 256.0 MB
     Current Mode: 1024 x 768 (32 bit) (60Hz)
          Monitor: COMPAQ FS740 Color Monitor
  Monitor Max Res: 1280,1024
      Driver Name: nv4_disp.dll
   Driver Version: 6.14.0010.8198 (English)
      DDI Version: 9 (or higher)
Driver Attributes: Final Retail
Driver Date/Size: 12/10/2005 06:06:00, 3955456 bytes
      WHQL Logo'd: Yes
  WHQL Date Stamp: n/a
              VDD: n/a
         Mini VDD: nv4_mini.sys
    Mini VDD Date: 12/10/2005 06:06:00, 3536768 bytes
Device Identifier: {D7B71E3E-4201-11CF-F47B-63E303C2CB35}
        Vendor ID: 0x10DE
        Device ID: 0x0141
        SubSys ID: 0xC3693842
      Revision ID: 0x00A2
      Revision ID: 0x00A2
      Video Accel: ModeMPEG2_A ModeMPEG2_B ModeMPEG2_C ModeMPEG2_D ModeWMV9_B ModeWMV9_A
Deinterlace Caps: {212DC724-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(YUY2,YUY2) Frames(Prev/Fwd/Back)=(0,0,1) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_PixelAdaptive
                   {335AA36E-7884-43A4-9C91-7F87FAF3E37E}: Format(In/Out)=(YUY2,YUY2) Frames(Prev/Fwd/Back)=(0,0,0) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_BOBVerticalStretch
                   {212DC724-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(UYVY,UYVY) Frames(Prev/Fwd/Back)=(0,0,1) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_PixelAdaptive
                   {335AA36E-7884-43A4-9C91-7F87FAF3E37E}: Format(In/Out)=(UYVY,UYVY) Frames(Prev/Fwd/Back)=(0,0,0) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_BOBVerticalStretch
                   {212DC724-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(YV12,0x32315659) Frames(Prev/Fwd/Back)=(0,0,1) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_PixelAdaptive
                   {335AA36E-7884-43A4-9C91-7F87FAF3E37E}: Format(In/Out)=(YV12,0x32315659) Frames(Prev/Fwd/Back)=(0,0,0) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_BOBVerticalStretch
                   {212DC724-3235-44A4-BD29-E1652BBCC71C}: Format(In/Out)=(NV12,0x3231564e) Frames(Prev/Fwd/Back)=(0,0,1) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_PixelAdaptive
                   {335AA36E-7884-43A4-9C91-7F87FAF3E37E}: Format(In/Out)=(NV12,0x3231564e) Frames(Prev/Fwd/Back)=(0,0,0) Caps=VideoProcess_YUV2RGB VideoProcess_StretchX VideoProcess_StretchY DeinterlaceTech_BOBVerticalStretch
         Registry: OK
     DDraw Status: Enabled
       D3D Status: Enabled
       AGP Status: Enabled
DDraw Test Result: Not run
D3D7 Test Result: Not run
D3D8 Test Result: Not run
D3D9 Test Result: Not run

-------------
Sound Devices
-------------
            Description: Realtek AC97 Audio
Default Sound Playback: Yes
Default Voice Playback: Yes
            Hardware ID: PCI\VEN_10DE&DEV_0059&SUBSYS_541A16F3&REV_A2
        Manufacturer ID: 1
             Product ID: 100
                   Type: WDM
            Driver Name: ALCXWDM.SYS
         Driver Version: 5.10.0000.5850 (English)
      Driver Attributes: Final Retail
            WHQL Logo'd: Yes
          Date and Size: 5/18/2005 20:50:30, 2319680 bytes
            Other Files:
        Driver Provider: Realtek Semiconductor Corp.
         HW Accel Level: Full
              Cap Flags: 0xF5F
    Min/Max Sample Rate: 100, 192000
Static/Strm HW Mix Bufs: 26, 25
Static/Strm HW 3D Bufs: 26, 25
              HW Memory: 0
       Voice Management: No
EAX™ 2.0 Listen/Src: Yes, Yes
   I3DL2™ Listen/Src: Yes, Yes
Sensaura™ ZoomFX™: No
               Registry: OK
      Sound Test Result: Not run

            Description: Modem #1 Line Playback (emulated)
Default Sound Playback: No
Default Voice Playback: No
            Hardware ID:
        Manufacturer ID: 1
             Product ID: 81
                   Type: Emulated
            Driver Name:
         Driver Version:
      Driver Attributes:
            WHQL Logo'd:
          Date and Size:
            Other Files:
        Driver Provider:
         HW Accel Level: Full
              Cap Flags: 0x0
    Min/Max Sample Rate: 0, 0
Static/Strm HW Mix Bufs: 0, 0
Static/Strm HW 3D Bufs: 0, 0
              HW Memory: 0
       Voice Management: No
EAX™ 2.0 Listen/Src: No, No
   I3DL2™ Listen/Src: No, No
Sensaura™ ZoomFX™: No
               Registry: OK
      Sound Test Result: Not run

---------------------
Sound Capture Devices
---------------------
            Description: Realtek AC97 Audio
  Default Sound Capture: Yes
  Default Voice Capture: Yes
            Driver Name: ALCXWDM.SYS
         Driver Version: 5.10.0000.5850 (English)
      Driver Attributes: Final Retail
          Date and Size: 5/18/2005 20:50:30, 2319680 bytes
              Cap Flags: 0x41
           Format Flags: 0xFFF

            Description: Modem #1 Line Record (emulated)
  Default Sound Capture: No
  Default Voice Capture: No
            Driver Name:
         Driver Version:
      Driver Attributes:
          Date and Size:
              Cap Flags: 0x20
           Format Flags: 0x0

-----------
DirectMusic
-----------
        DLS Path: C:\WINDOWS\SYSTEM32\drivers\GM.DLS
     DLS Version: 1.00.0016.0002
    Acceleration: n/a
           Ports: Microsoft Synthesizer, Software (Not Kernel Mode), Output, DLS, Internal, Default Port
                  Realtek AC97 Audio, Software (Kernel Mode), Output, DLS, Internal
                  Microsoft MIDI Mapper [Emulated], Hardware (Not Kernel Mode), Output, No DLS, Internal
                  Microsoft GS Wavetable SW Synth [Emulated], Hardware (Not Kernel Mode), Output, No DLS, Internal
        Registry: OK
     Test Result: Not run

[please use http://pastebin.com to host your dxdiag report please] - Jag100

Have you tried messing around with the LOD options, you can customize your graphics, in particular, turning off reflective water helps the frame rate.I sometimes take the Supports bar down to Zero, cos i use the custom steelworkz package.I also open my park from dawn till about 6-7, this cuts out nightime, which puts a strain on the system.Try these and see if it helps.ALSO, dont have MASSIVE amounts of water.

I cannot see why my computers cannot handle the top end facilities of the game and I have actually FIXED the problem. I have got rid of that major pile of memory eating crap known as WINDOWS XP and gone back to Windows 2000. The game is running like an absolute dream.
Same Hardware, same drivers and everything else.
Goes to show doesn't it?
Anyway guys thanks for the help and suggestions.
Regards

Okay basicly the lag is not going to go, At some point in the game you will get it. Others more than others.

Try updating your drivers and also try using the auto dected settings in the RCT game options menu

I done some experimenting with CPU / Graphic bottlenecks while playing RCT last year. Check this thread:

http://forums.rct3x.net/showthread.php?tid=1086

Like me, you may find the results supprising. I no longer have the PC used back then, and have a Dual Core AMD now, and as far as RCT goes, the dual core is wasted, as RCT:3 cant use it.

It's like this, my CPU has 2 cores at 2.4 Ghz each, and yours has 2 at 1.7 Ghz. If RCT was able to run 2 threads at once, you would see it utilise the CPU to 100% (Giving me 4.8 Ghz, and you 3.4 Ghz), in my system (and you will see on yours), CPU utilisation is always 50%, so RCT only plays on my CPU at 2.4 Ghz, and on yours at 1.7 Ghz, leaving half your processor to take a break.

It's better in RCT to run a very fast single core CPU, rather than 2 slower processors. Dual core is getting more common tho, and games will support it soon.

I suggest you read up a bit more on the Intel Dual Core technology. Actual core speed for each is 3.4GHz and benchmarked out-performed an overclock processor (7.2GHz), not quite as good as two seperate processors but better than one. With the set affinity options the game is running on one core and everything else on the other. Top stuff.
Anyway problem is solved, I would recommend to anyone to use Windows ME or Windows 2000 to get the best out of this game, Windows XP (esp PRO) is too resource hungry, why else is the minimum spec for RAM double that of any other operating system?
I have a huge park, graphics pushed up full, loads of water with reflections etc..... over 3000 guests and it is a smooth as a babies bottom. Nuff said?

well thats OK

so as this is solved i will lock the thread