PC shuts down randomly and boots up again

Hello everybody,

I was playing a game and suddenly pc turns off and goes back on. I was like, okay then..
A month later (now) it happens again, this time while just randomly browsing facebook.
I've tried booting it in safe mode but it happens again. Im not a computer expert so I dont know how to fix hardware problems (cuz it's probably the PSU)

I've overclocked my GPU once but I figured it didnt improve much so I returned it back to the normal settings in Afterburner. I did not reset my motherboard cuz I dont know how to safely do so.

My pc specs:

Processor
Intel(R) Core(TM) i5-3550 CPU @ 3.30GHz
Graphics Card
MSI R7850 Twin Frozr
Power Supply
MS-Tech MS-N750-VAL 750w

It's not overheating cuz I checked the CPU temps aswell as the GPU temps in multiple high end games and the temp was around 50 max.

How do I fix this problem? It's driving me nuts! I just wanna play some games..

Thanks in advance.