My PC shuts down randomly

[Agent Dankwalker]

I've built my PC over 2 years ago and it worked fine until about a month ago.
My PC shuts down randomly and when I mean randomly, it really does shut down randomly. Today it shut down 3 times but yesterday it didn't at all. It shuts down in basic tasks and sometimes while gaming. The way it shuts down is, the screen just goes black, then the OS starts to reboot. The PC sometimes keeps shutting down while rebooting xD.

My specs
CPU: AMD A6-6400K 3.9 Ghz overclocked to 4.02 Ghz the day I built it
GPU: AMD Radeon R7 250 2GB of VRAM
PSU: It's big enoguh, can't remember how many watts.
RAM: 8GB ( the ram speed is identical on both sticks )
HDD: Western Digital 1TB

Also, I thought it might happen due to overheating but I ran FurMark, the fans were on 100% and the GPU temperature hasn't gotten over 40C but it has shut down halfway through Furmark. The temperature didn't go over 40C because A: the GPU isn't super powerful and B: I have 5 big ass fans.
I keep my drivers always up to date.
I think the only problem might be the PSU.
I was thinking it was Windows so I reinstalled it and the problem is still occuring
Also tried applying fresh thermal paste.
Thank you in advance for your help

 

[RARRAF]

well the first thought would be the PSU but you do not know so ?????

 

[Agent Dankwalker]

Well my first thought is the PSU as well but the thing is I just gave it to a friend of mine to put it together, I just picked the GPU and CPU that I wanted at the time.
He's on vacation so I'm possibly trying to find a solution until he comes home which is gonna take a week or two.
Also I've found that if I shut down my PC for like about an hour, then it just works for like a day and then starts shutting down again.

 

[Agent Dankwalker]

I've made a small breakthrough, I believe after searching the net that the real problem is the either the GPU or RAM.

Reason why I think it's the GPU is that sometimes when I play games, before the PC reboots and shuts down is that the screen goes red and just stays like that for 20 seconds with very distorted audio.

Reasony why I think it's RAM is because sometimes when the PC shuts down, I try to boot it up but the PC keeps shutting down while starting up and goes in a loop hole and can't even get to BIOS. When this happens, I try about 5 times to boot it up and eventually am able to but that is extremely annoying and frustrating.