Question PC shutting down at seemingly random moments (see details)

[Hwoar_ang]

Hardware:
CPU: Ryzen 3700x
GPU: Asus RTX 2080
RAM: GeiL DDR4 Evo Spear (2x8) 3200
MOBO: B450 MSI gaming plus max
PSU: Coolermaster MWE gold 650

Problem description:

1. PC is just shutting down. (no blue screens)
2. This happens both in idle and during gaming.
3. I noticed that the USB ports still have power after this shut down.
4. I can't use my power button to turn on the PC, I have to make sure the PC is powerless first and then it boots again.
5. Temperature's are fine under load.
6. This weekend it worked completely fine for around 32 consecutive hours (heavy) gaming load, then after 8 hours idle the problem happened again.

I would appreciate it if anyone has some insight on this. Can we rule out some parts already? I am able to try a different PSU next week.
Testing things is a bit of a nightmare, because the current system can work fine for longerish periods as well.
I am thinking that it may be the motherboard, but thats hard to test / prove. It is also going to be hard to explain this to the store.

 

[Ralston18]

How old is the PC? Has the case been opened for cleaning and reseating cards, cables, RAM, etc.?

If not, do so.

Use Reliability History and Event Viewer to look for error codes and warnings that correspond with the times of the random shutdowns.

Use Task Manager and Resource Monitor to observe system performance leading up to the shutdowns.

Any problems after booting into safe mode?