Random Shutting Down

i just built a new gaming pc for my brother the other day and now it freezes and shuts down with out warning. i ran intel burntest and it was fine on maximum. but when i ran unigine heaven benchmark it would just freeze in the first couple of seconds of benchmark. This card he has got is my old one and it ran fine on my rig using ultra on unigine valley but on his it crashes. any suggestions please? 

all vga drivers are updated.

i3 3220

gtx660 gigabyte windforce edition oc

gigabyte ga-b75-m-d3h

2x4 gskill ripjaws 1600mhz

1tb wd hdd 

aerocool vp-550 also was mine and ran perfectly with my 3470.

card isn't that old. probably drivers