I had the same problem with my system when I first built it. It turned out to be the processor overheating: my BIOS would automatically reset the pc when a certain temperature (around 55c) was reached. A little better cooling in a better designed case sorted my problem out. I now have Motherboard Monitor running full time, as I am paranoid about high temeratures, and this prog keeps me up-to-date with my system temperatures. Dunno if this is the same as your problem, but maybe it`s worth checking out, if only to prolong the life of your CPU