PC turns off randomly then have to wait for it to turn on

[Exodias3219]

Hi, earlier this morning I played for like 5 minutes then my PC suddenly shut down itself like no power at all. I tried to turn it on afterwards but it doesn't turn on at all. After resting for 30m-1hr tried to turn it on and then it worked. Temps are okay, newly replaced thermal paste. Only the GPU is overclocked. I also noticed that BIOS settings have changed, reverted back to defaults. Before writing this post I tested the PSU by shorting out the 3rd and 4th pin(PSU-ON) and then it worked. It also happened again this noon while idle. I wonder if it's the motherboard or the PSU has problems. I updated the BIOS last 3 weeks ago. Added a 4gb Avexir RAM yesterday(temporary, borrowed). Also tested the power cable and AVR, all good.

Processor: i5 3470
MB: Biostar H61MLV
RAM: 8gb HyperX Fury + 4gb Avexir
Gpu: GTX 1050ti
PSU: Seasonic M12ii EVO 620watts
Ssd: Samsung 830 128gb

Game: Assassin's Creed Origins

 

[R_1]

"Before writing this post I tested the PSU by shorting out the 3rd and 4th pin(PSU-ON) and then it worked."
not a test really as it cannot tell you if the PSU can support any load but itself. the test shows the PSU can power ITSELF and nothing more. it is not cleared as functional.

the BIOS settings indicate the battery on the motherboard may be failing. replace if you can.

can you borrow a PSU for testing?

 

[Exodias3219]

Well sorry about that, I thought that when the PSU turned on it means its functional.

I'll try to replace the motherboard's battery.

No, I can't borrow a PSU since my friend is out of town atm.