Computer crashing/freezing/rebooting when playing intensive games

[Mikoloco]

Specs:
HD 7950
FX 6300
4 GB RAM
760 GM P23
PSU LC POWER 6600 600 Watts
So, yeah, this is happening in games like Far Cry 4 after some playtime, for example after 30 minutes and yes my temps are very good (54c on air at most due to the cold weather in my country)
CPU doesn't go over the 55c neither.
I ruled out the GPU being the problem because I ran 3D Mark 11 and got the average score for my card.
I ran my memtest86 and RAM has no problems.
I also doubt it's the PSU considering how I used a GTX480 before this which had an higher TDP than my HD 7950
Basically what I suspect that is the problem is either one of my two hard-drives or the fact that I didn't really reinstall windows after getting my new HD 7950.

 

[Mikoloco]

On a related note Watch Dogs run perfectly fine for hours and hours with everything on ultra except the textures.
But when I turn the textures to Ultra my computer reboots by itself in like 10-20 seconds.

 

[TechGeddon]

when you play games does the computer go black screens / no signal but you still hear sound and such?

 

[Mikoloco]

when you play games does the computer go black screens / no signal but you still hear sound and such?

Yes, exactly that!

 

[TechGeddon]

Your video card istill not getting enough power. It can handle easy stuff ok. But when you go hard it draws alot of power... suggest trying a psu swap with something that has more amps on the 12v rail than what yours has.