+1, Sudden reboot is often because your hardware is overheating, Almost of time, it's your processor, is it overclocked ??, the fan is it working ?, you can do monitoring with "SpeedFan", or other little software,
Maybe it can be your graphic card, but in most case it only freeze the computer !! and you can see a lot of glitch in your screen (polygon) 1-2 minutes before freezing !!!
the last thing i can see is your powersupply is damaged, really dirty or inapropriate for your hardware,