PC Freeze/Crashing while Gaming-temps ok.

[goof5150]

PC will crash 10min in playing MKX. 1 out of 5 times it will run fine for over an hour or so. The game will freeze and nothing works. It also crashes when i play Witcher 1, although this happens about 45min in or so and it reboots when playing this game. No error message and Nothing logged in event viewer.
GPU temp high 65c using OSD MSI afterburner
CPU temp high 56c using coretemp
In the process of running Memtest for last 2 hours and no errors so far.
I have tried swapping GPU and appropriate drivers and same issue.
specs:
i5 3470 3.20 ghz
4GB RAM
Mobo: B75m-d3h
GPU: ASUS 7970HD (also tried GTX 650ti)
PSU: Corsair CX600
The issue is new to me because i never attempted to run a game on my PC before. I purchased the CX600 a few days ago and installed the 7970. Previously The pc had a generic 350w psu and a 650ti but i never ran any games on it.

I have not ran any stress test but plan on doing so after a few more hours of memtest. Also I disabled auto shutdown option in windows but have not played witcher since.

Input is welcome and you're patience is appreciated.
Thank you.

 

[goof5150]

So I ran prime95 blend test and somewhere within 2.5 hour mark it restarted. No error in event recorder. Cpu temp was 72c at the highest I saw.

 

[goof5150]

So I forgot to mention that there were no errors on the memtest i ran for 7 hours. I attempted to run prime95 blend test a few more times and watch the temps carefully to see if they were too high before restart. The temp was fine but it will still restart within 30min every time.
I took out the single 4gb ram stick and replaced it with a new same model single 4gb I had which I planned on using to upgrade to 8gb and am running prime95 blend test on it now, it has been running for 2 hours now. I hope the ram was the problem.