Black screen and pc restart problem

[ozm8ey]

Hey guys I'm having some trouble with my pc. Basically what happens is at random usually only once a day my pc screen will go black and say powering off then my pc will restart, but its not like an instant restart like when your psu is faulty. I tried to stress test my gpu by playing some forza horzion 3 and forza motorsport 7 but for some reason it doesn't happen when i played those. Last time it happened i was just using the internet when all of a sudden the screen went bacl then pc restarted.

I checked temps and seems quite normal even on a 43 degree Celsius day like today my cpu was only at 35 degree's Celsius
I'm hoping its my ram not the GPU, any help would be good, thanks