Hi there from Australia, I need your help please!

I have made a homemade CNC router/mill. All works well except for this persistent problem with the stepper motors locking up on me and missing steps. It is okay at the slow speed settings, but even then you can hear it occasionaly making a "knocking" noise. From mid range to full speed it makes a constant "knock'' or "ticking" noise at about one second intervals, followed by a " lockup" every 20-30 seconds of run time. Apart from this the motors sound nice and smooth.
I have disconnected the stepper motors from the machine and still the same problem, so it is nothing to do with friction etc.
I have tried everything i have read in these forums but to no avail!

My computer is 1.8Ghz Pentium 4, 1 Gig ram, 16meg plugin video card, Windows 2000 pro. running Mach3 and absolutely nothing else. No antivirus, internet, or any other program is installed.
I run 3 x Gecho 251s on heatsinks through a Opto-isolated BOB.(not cheap Chinese stuff), with power supply and motors to suit. All cables are shielded and ground appropriatly. I have tried the following:

1) re-installed Win. 2000 and latest drivers.
2) reinstalled Mach3, and tried the latest version.
3) Installed Windows XP. & service pack 4.
4) tried optomisation, special driver, 1/2 Sherline mode, various step/direction pulse widths, enhanced pulsing, turned watch dogs off/on, tuned plug-ins off, played with Dir low/active etc. kernal speeds and everything else i have read in these forums.
5) I am an electrician by trade and have checked all connections and ground everything that needs to be.

I have a "hunch" it is the computer or parralell port card putting out a ****ty pulse stream, but i'm open to any suggestions at all! Is it something deep inside the 'Nuts & Bolts' of the operating system that can be interfering? Money is in short supply so i don't want to spend any more( the missus will flip!)
Please put your 2 cents worth in, i'll try anything

Failing this, what do you guys think of trying a Smoothstepper (usb) solution?

Kind regards, Julian.