If it shuts off and stays off. Your overheating.
If it reboots, its more than likely a Driver issue.
Check your drivers here:
http://www.Driveragent.com
In the meantime go to your my computer icon, right click, click properties, go the the advanced tab. Then click Settings under STARTUP and RECOVERY. Unclick Automatically restart.
Next time it happens you should get a blue screen, post it up..the error keep a piece of paper and pen handy.
copy down all the numbers thatt look like 0x000000000