This is a horrible longshot.
I had an issue not long ago, on my all-AMD desktop. It would randomly reboot, though would never freeze, as yours. And I never run YT videos more than 30 or so minutes. I was on the verge on buying a new (miniPC) because I was convinced my aging desktop was just that - too old 
Anyway, I spent about 8-12 hours intensely researching and came across a somewhat old article, where there is this absolute deeply technical person (name is Rich), who suggested a couple of settings (not for me, but for that thread, but I used).
I applied the boot entries - problem solved … I have NEVER seen the issue since. My orig problem obviously is the result of a kernel update, or some other low level update.
Anyway, it’s an extreme longshot, but here’s what I added to the grub boot line
amdgpu.ppfeaturemask=0xffffbffb
amdgpu.dpm=0
Here’s the thread I found, with the solution (for me)
https://bugzilla.kernel.org/show_bug.cgi?id=206903
See post #33 for explanation
And see 50 for other info … last post is 150, and yea, I read each reply