Well, I think I can definitely say it’s a problem with Unreal II itself now after spending an evening doing experiments.
I ran the program on its startup screen until it started to show the juddering
motion and timed it doing so. It took approximately 15 minutes.
I then tried downgrading various settings, such as video card clock frequencies,
AGP down to 4X and RAM speed down to PC2100.. None of these had any effect.
Also, exiting Unreal II at the jittery stage and re-entering it imeadiately
worked and the jitters didn’t reappear for another 15 minutes. If it was
a thermal problem the fault should have reappeared quite quickly.
I also did an experiment by running the program in windowed mode showing
CPUidle and the motherboard monitor programs in the status bar. The CPU was
never fully utilised (it peaked at about 93% utilised) and the CPU temperature
never went above 44C. (The case temperature was 37C, apparently).
Now, all I have to try tonight is downgrading my NVidia drivers to the version
3.x.x as in the Unreal II FAQ it mentions problems with the new unified drivers.
If this does nothing I’ll write a bug report.