The only times I have ever seen a PC 'auto-restart' while playing any game has been due to heat-related issues, as in the PC I was playing on did not have good enough cooling for the hothot vid card and other system components...it would heat up to a point where it simply shut down and rebooted.
You might try opening the PC case and aiming a floor fan directly into the case...if your PC stops rebooting with more air-flow then the problem is likely related to insufficient cooling.
Additionally, some folks have mentioned an underpowered system can cause similar reboot issues, but I have never personally seen this. Nonetheless, it makes sense that if your system runs a pretty newish hot-rod vid-card, 2 HDD's, 2 optical drives, a floppy, a web cam and a USB cup warmer
, a 300 Watt power supply may not cut it. Make sure that you have a good-quality power supply of sufficient (350w and Up!) wattage for all your stuff.