I have the same problem
I have had the same problem for quite a while now. Despite many many attempts to figure out what is causing the lockups, I still cannot find a pattern. I have seen lockups under KDE 2.x, KDE 3.x, and Enlightenment. At first I suspected AGP bug since they seemed to happen when I was using up a significant amount of my system resources, but now I get lockups even when my computer is idle (go to sleep, wake up, find funny vertical line pattern on screen, sys doesn't even respond to SysRq). I am running the latest nvidia drivers (build 2960), but I have had these lockups for several versions of the driver, and no-one can help...they all blame the AGP bug (I tried several bugfixes just in case...a few actually made the problem worse).