It might be due to your CPU overheating. Make sure fans are on and in proper places.
You could have a faulty powerbox. The motherboard 'standby' light kept flashing (although it seemed controlled) but nowhere in the manual did it say anything about what a flashing standby light meant. It restarted itself every 10-15 minutes.
Try booting up, and entering the BIOS setup, and browsing around for CPU temperature. You may be able to find an indicator of its temperature from there. Otherwise, just leave it in that for a while for 20 minutes, and see if it restarts itself.