I think I discovered the problem...much to my shock, when I rebooted into Windoze and tried UT2K3, the exact same thing happened....so I cracked the case and lo, the stupid fan on the card isn't spinning very fast...I think it's about to die (the blasted card is only 1 month old or so).
This is likely the problem. It's not a Linux thing, it's my GF4 card overheating...and of course, it's a VisionTek! Which means since they went out of business, no support.
Thanks for the advice anyways!