Video Reboot Problem...Please Help

A couple of questions rolled into one problem. I just built a system for a friend (specs at bottom) and every time some sort of 3d rendering occurs (ieiablo II, 3dMark 2001Benchmarking) it will run for a minute or two, then reboot the computer. At first I figured DirectX, or bad video driver, but have almost ruled these out (unless someone can tell me differently). Part of the wierdness to this is in 3DMark. If I run the demo, not the benchmark, it plays beautifully. No problems at all, but the benchmark will reboot the computer.

Specs: Asus P4G8X
GeForce Ti4600 with 41.09 drivers
DirectX 8.1
WinXP
Audigy 1 Platinum
P4 2.4 GHz

If anyone could tell me how to make the computer stop rebooting during 3d processes/rendering I would be most appreciative.

Tech Support: "Hello, tech support, may I help you?"
Customer: (in a thick Russian accent) "Yes. Monitor is working fine but has sparks and smoke flying out back. Is ok?"

Turning down the Hardware Acceleration has helped a lot. Random freezes still occur during 2dMark 2001, but it kicks back in and continues. There is no reboot anymore though. I also turned off write combining. I have turned the Hardware Acceleration down three notches from the right. Any more and 3dMark will not run. Does this help with suggestions? Does anyone know where to turn off FastWrites for the Asus P4G8X? Bios? Windows? Again, I am running a Ti4600. Need to turn off fastwrites. Please help.

Tech Support: "Hello, tech support, may I help you?"
Customer: (in a thick Russian accent) "Yes. Monitor is working fine but has sparks and smoke flying out back. Is ok?"