Which AGP handler code are you using, nVidia's or the kernel's agpgart?
You choose between them in your XF86Config-4 file, with Option "NvAgp" "<number>", where <number> goes from 0 to 3. 3 is the default if there is no option specified, and 3 is what I run my system at (same motherboard chipset as you). If you're using anything else, you might want to try 3.
Otherwise, if you're passing the option "NVreg_EnableVia4x" to the NVdriver, see if not doing that helps at all. Unload the driver, change the system (modules.conf file) so it doesn't pass that option, and reload the driver, and see if that helps at all. Although I do pass it, and it works just fine, so I'm not sure if it'll help or not.
When it locks, double-check the temperature of the video card (I assume the fan will still be running? Just feel the temp of the air it's blowing, as compared to normal use). I've seen a couple times where that can cause lockups, even if the CPU is cool.
Edit: Does the UT 2003 demo still work fine? Could you install it again to check?