As always it could be anything. You have to rule possibilities out one at a time.
When the system drops off do you see lights flicker, fans, heaters, air conditioner turn on, etc. If yes it is your incoming power. You can try a better power supply, an uninterruptible power supply or maybe a different outlet. For a si,ple test try taking out anything in the system oyu don't have to use. Extra drives, second stick of ram, add-on cards etc. This reduces the loa on the pweor supply which might help a little.
Restart and check your system and CPU temperatures in bios. CPU should be less than 60 deg C (preferably <55) after high work load. Below 50 when just surfing. System temp should be <40-45 deg C.
Do a complete virus scan using an antivirus with the latest update.
If anything is overclocked or set for "performance" take it back to standard. See if the problem goes away.
Disconnect everything in your system but 1 stick RAM, 1 HD and video. See if the problem goes away. If not try a different stick of RAM. If it still doesn't go away it's MB or CPU, maybe HD but that's very unlikely.
If the problem does go away start putting things back in one at a time until the problem comes back. When it does you found the guilty party.
My worst nightmare was 4 months of crashes. Thought I had tried everything. When I decided to upgrade my sound card the system became totally stable and has been for 10 months now.
