Help! Computer randomly restarting when playing random games

[Xhosa]

Please Help I recently built a gaming rig around April of last year and my computer seems to randomly restart when playing some games.
It will restart after about an hour or two of playing certain games like csgo.
While restarting almost instantly when trying to play a game like outlast.
However I can play WoW or Far Cry 3 for hours with no restart at all.
I have done Multiple test such as Memtest86,Prime95,furmark,heaven'sbenchmark and these can run for hours with no hiccups at all.

Rig :
Windows 8.1 Pro
MSI Z77A-G45
i7 3770(non k)
4 x 4 at 1333 gskill ares
gtx 780 msi twin frozr
hx750
2tb hard drive
3tb harddrive

I suspect that it may have something to do with the psu gpu or hard drives. This was my first build and during the creation of the computer I took the serial number off of the gpu.

 

[SamiSC]

are you experiencing any over heating with the CPU or GPU, check the temps while you gaming to see if that is the issue.
check if your Hard drives are in good health and not making any clicking noises and same goes for your power supply.

 

[Xhosa]

are you experiencing any over heating with the CPU or GPU, check the temps while you gaming to see if that is the issue.
check if your Hard drives are in good health and not making any clicking noises and same goes for your power supply.

The temps seem fine the card is suppose to be able to operate up until 92 degrees and it never goes above 75 in furmark, and its running at that temp for hours in games where it restarts the temps are between 50 to 68 degrees.
How would I check to see if my hard drives are in good health?

 

[Xhosa]

are you experiencing any over heating with the CPU or GPU, check the temps while you gaming to see if that is the issue.
check if your Hard drives are in good health and not making any clicking noises and same goes for your power supply.

Also when loading certain programs and games the hard drives don't make a clicking noise but a sort of coil whining similar to that of a gpu, is that normal? Btw thanks for the response

 

[SamiSC]

Could be that when your gaming, the GPU needs more power and your PSU cant support it. although the HX 750 should be fine for any single GPU, it could be having problems, but I personally dont know how you can check for faulty PSU.

 

[Xhosa]

It could be the psu, that is a possibility, but what about the benchmarks and testing if the psu can handle the 780 while at full load for hours on heaven's benchmark and furmark, why would it not be able handle games that stress the gpu less. In csgo for example the game shuts down within a match or two but I can play WoW or Far Cry 3 for hours upon hours with no hiccups.