Question About once per week, display goes black and all fans rev. Plus, sometimes can't switch off with power button?

Dec 30, 2020
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Hi, I've been struggling with this issue since about 2 months after I built my PC, back in August.
The issue was initially that every now and then, the monitors will just go black, while all the fans continue to run. The GPU fan is usually not running, as it happens when the system doesn't seem to be under any particular load. However, when this happens it starts running. I would have to use the power button to switch it off.
This kept happening, infrequently enough that it is impossible to test any parameters but frequently enough to be concerning. And, after a few times, the power button stopped turning the pc off. I have no idea how this is possible, but I started having to use the power switch on the back of the PC.
When trying to solve the problem, I checked my temps and realized that my CPU temps were wayy too high, and I realized I hadn't screwed the cooler on enough (oops... it's my first build so I didn't realize). I screwed it on better. To my suprise, this actually seemed to fix the issue, it didn't happen for a few weeks.
Recently I moved house, and took my PC in the car. Now, it is doing it again! It has happened twice this week, once without the power button working and once with. I have no idea which component could be causing this, since it seems to be affecting so many entirely different parts.
Anyone got any ideas? This is my only computer and I am a student, so any RMAs would put me out of work for the time that it took to get the replacement. Therefore, I want to at least be quite sure that it would fix the problem.

Some notes:
I have 2 monitors, and it affects both, so it can't be the monitors/cables.
I don't have integrated graphics or access to any alternate parts, so I can't try that.
 
Dec 30, 2020
3
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Ok, so, the situation has changed, and got worse. I put my PC into sleep mode yesterday, and when I came to start it back up again, it didn't. To be more specific:
  • PSU fan started
  • Unclear whether HDD was running (not sure how to tell this one)
  • Lights on mouse stayed on (they are on during sleep mode)
  • LED on GPU did not switch on
  • Case fans, GPU fan, CPU fan all did not start
  • Power button lit up, but could not be used to turn it back off again.
I turned it off at the switch, turned it back on again and still nothing. Switched it off, poked the insides a little bit and it switched back on. But, now, it did it again (after sleep mode once more) and I can't get it to start this time. :(
Again, the issue has gotta be with the PSU or mobo. Any insights would be appreciated.
Here is my full parts list: https://uk.pcpartpicker.com/list/pgGGWb