"If it is rebooting without a BSOD or other error being displayed, you could investigate the Windows Crash Recovery settings which may give more clues. I had nVidia driver issues once which were causing exactly these sort of random reboots.
Right click on my computer > Properties > Select the 'Advanced' tab > and under 'startup and recovery' click the 'settings' button. This will open a new window, and under the 'System Failure' heading, uncheck 'Automatically restart.'"
I had this happen to me just this weekend. It turned out to be my video card. The card was not overheating or anything apparent. I swapped out the card for an old one I still had and all the issues stopped.
