Random crashes and restarts. Please help me fix this for my pal <3

Goodafternoon all.

So long story short:
Im helping a friend fix his pc we build together.
The specs are as followed:

OS:Windows 10
CPU:i5-6600K
Motherboard:ASUS Z170 SABERTOOTH MARK 1
Memory:16Gb Dominator RAM around 3000Mhz
Graphics Card:AMD R290
PSU: Seasonic 620W bronze

Everything seemed to word, but he got crashes and restarts at very random intervals.
Sometimes right after startup. Sometimes after 2 hours of gaming. But it did happen more often the longer he played.
After a big crash he didnt have a video signal anymore, so we assumed the gpu broke. It does infact not work anymore, but after removing it it still crashes from time to time.

I did a stress test and a health check on the cpu. That was a-oke. Temps are good all around. I changed ram sticks to see if it had a faulty one. Also not the issue.
I flashed the BIOS which went well, but it did not solve my issue.

So i'm kinda at a loss what it could be. Im guessing either a PSU issue, or something with the motherboard. I will switch the psu with the one I have to see if it helps.
Any advise? Also, the R290 doenst work anymore. Could the issue be that it is a legacy bios card and I have a UEFI bios on the motherboard?

I hope we can crack this puzzle. My thanks in advance!!

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Alphasnor said:

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Could the issue be that it is a legacy bios card and I have a UEFI bios on the motherboard?
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Welcome to TenForums.

My first thought after reading this is that if you physically modified the motherboard (removed the stock BIOS, and put in a different BIOS) then that could be the reason.

My second thought is, yes, try a different PSU with higher wattage though. (Be careful though, if my first thought is true, I don't know about a fire hazard or anything like that)

Alphasnor said:

So i'm kinda at a loss what it could be. Im guessing either a PSU issue, or something with the motherboard. I will switch the psu with the one I have to see if it helps.

For me, it seems to be power related, PSU and motherboard being the the two primary candidate. Also, the power coming into the PSU could cause similar issues, via spikes/brown outs. I'd start with either move to a different outlet, or add a UPS to the current outlet, if there's none.

The power outlet in the house seldom is just for the computer, there could be other devices within the same power circuit connected to the same breaker in the breaker panel that occasionally draws power close to the breaker size. Like fridge, coffee machines, washers, dryers, etc.