As far as I know, if there's no bsod it's pretty much guaranteed to be a PSU problem

There's an option in Windows settings (advanced system properties, near where you change page file settings) to disable auto restart and actually show/log the stop code. Disable the auto reboot, fire up something like Furmark to stress the GPU and PSU and see what happens

I had a similar problem a while ago.

Do you have this error in your event viewer?

The solution for me was CPU replacement. AMD sent a new one pretty quickly though.

What does windows event log say. If you get a bsod or an error code it will be stashed on your logs.

Monitor temps? You should notice slowdown due to throttling before a shutdown if that's the problem.

That is identical to the problem I had on my Ryzen 5 1600. There were two problems. 1. PSU was overloaded. 2. The CPU was overheating (probably because I was pushing it a little too hard and/or because the thermal paste had dried a bit and cracled).

You say you have 4×8GB RAM. I had an issue where I had 2×16 GB for sometime. Later, I wanted to upgrade and got another 2x16 GB of the same RAM for a total pf 4×16 GB and my system started to randomly reboot. Taking out the two new RAM sticks stopped the random reboots. Some RAM manufacturers recommend using RAM from the same package (i.e., for the same 4x8GB, all 4 sticks should be packaged as one). If the above is your setup, I would start by installing the RAM that came within the same package and go from there. If you want to have 32 GB, I would recommend installing 2×16GB. Also ,keep in mind that it is highly unlikely that your CPU and/or motherboard supports quad channel RAM evening you have 4 slots for RAM.

I know you said it's not the ram... but I had a similar problem. The ram would pass tests, but the computer would still reboot. If I went to one stick of RAM it wouldn't reboot.

Turns out I just need to up the voltage for the RAM.

Anyway, good luck

Could also be the motherboard or a loose cable/connection. Have you monitored system temperatures while playing?

Anecdotal, but I've had three occasions where such an issue was caused simply by a faulty SATA cable that intermittently disconnected the OS drive at seemingly random intervals depending on environmental factors. One of the easiest things to test and saves a lot of time troubleshooting if it happens to be the issue.

Might be worth a try to reinstall windows (or install windows on a different drive) to find out if it could be your windows install thats corrupted

I had a similar issue of intermittent and unpredictable reboots which was solved by replacing the CMOS battery on my motherboard.

If it's turning on and off randomly, I had the same problem due to the power button/cable. Try setting the power button setting when pressed to do nothing, and see if that fixes it while playing. If it does, then that's your cause, and replace the power button and cable. Thats the best thing I can provide, if it's not that then idk I'm not much of a computer person.