[SOLVED] Has my PSU or Motherboard become deffective?

[Terminasitor]

So here is my problem...
About two years ago I bought a used PC and an old 750ti and turned it into a "gaming computer" for my sister. She used it all this time with no issues (admittedly light use). However, recently I was doing some work on it (Winrar Compression) and suddenly the whole system shut down -not even a blue screen-. I tried rebooting but there was no display out, I cleared CMOS and took out the video card -to boot with onboard graphics- but still nothing happened. Thankfully I had a different compatible CPU (i5-4430s) and I was able to boot with that. When in windows I downloaded OCCT to stress test the system and about five minutes in the CPU test the same thing happened. Again I couldn't boot until I swapped the CPU.
I'm afraid that either the PSU or the Motherboard is bust since the problem persist despite the CPU change.
What do you guys think?

The system specs:
CPU:i3-4130
GPU: GTX 750ti
MB:Asus B85M-E ->Bios Version 3602(Latest)
Ram: 8gb DDR3 1333MHz(4 sticks of Kingston KVR113D3N9/2G)
PSU: 350Watt FSP350-60APN

 

[Eximo]

Hard to say. Clearly the board is capable, I imagine the CPU swap just took long enough for the PSU to cool down and reset itself. You should start there.

Worst case you end up with a new PSU for the next build.

 

[DRagor]

I'm afraid that either the PSU or the Motherboard is bust

Your assumption seems to be correct. I guess you do have a spare PSU for a test. If problem stays with a known working PSU it would point towards mobo. Of course theoretically it could be GPU as well, but nothing you were doing while shut downs happened was stressing GPU so that's unlikely.

 

[Terminasitor]

Your assumption seems to be correct. I guess you do have a spare PSU for a test. If problem stays with a known working PSU it would point towards mobo. Of course theoretically it could be GPU as well, but nothing you were doing while shut downs happened was stressing GPU so that's unlikely.

I don't have my own PC with me at the moment but when possible I will get its power supply and see if that's the problem. Also I really doubt that the GPU is at fault, since when the shutdown happened, during the stress test, I was on onboard graphics (I forgot to mention that before).