My Computer shuts off 10 mins into gameplay

hi all, no doubt this question as been asked many times and if i've not read all posts then i must of mist the answer, my problem is when only playing games downloaded from steam this only happens roughly 10 mins into game play... its awful and worrying as it is like someone just pulling the plug out from the wall.....:confused here is my spec

win 7 64bit - all drivers are up to date
700watt PSU
rampage III gene M/B
8gb corsair ram
60gb kingston SSD (for O/S only)
500gb internal harddrive
i7 960 cpu @3.2Ghz
1x artic freezer cpu cooler
1x case fan
1x external dvd rom drive

cpu-temps 30c(stock) ands 50c when playing games

thanks

 

Great machine and temps for the CPU, how's the temps on your graphics card?

 

thanks for the reply.. it runs at 65c when in full game mode, but i have many games that i play for hrs that have no problem, its just when i play games off steam..!!