How can i fix my computer from stop restarting during gameplay?

I recently purchased a Gigabyte Nvidia 9800Gt. Ever since then when ever a play a high end game ie assissins creed,cod4 on high setting (1920x1080) it always restarts about 10 mins in game. It works fine for the 1st 10 mins. I can play games fine on low settings as well.



my 1st though was a temperature problem, so i installed 2 fans, prob still occurs.



i also changed my power supply from 350watts to 650 watts, still no luck,



i've also installed the latest nvidia drivers and still no luck.



any suggestions/help?How can i fix my computer from stop restarting during gameplay?
get a new motherboard.



Also this could occur if the video card overloads and crashes but that's unlikely because its new.



It might also be a software issue causing your computer to automatically shutdown.