Your comp resets when its under load, right?

The first that comes to mind is the power supply, thats pretty often the problem like you described. I have herd someone had a 550 wat PSU and the comp restarted when entering 3d interface (when under big load), and he changed to a different (better quality) make 300wat PSU, and that did the thing. The PSU was so bad overrated, that it was actualy a shit.

Try setting the clock speeds in bios back to normal, even better under normal, and all the settings to minimal stress for your comp first.

You should have started by posting the specs - MainBoard, video, CPU, ram.

Was your comp running normal earlier? When did the problems start?

Most important, dont start playin around with your memory timings, set all to normal, or below. You can Tweak after all is working normal, not earlier.