(In reply to Shiverly from comment #0)
> [...] dmesg has errors. 

I only see messages about failing to allocate a larger BAR, which is harmless.

> I think this is either AMDGPU bug or something ryzen related (most likely
> not, because they manifest as sudden reboots, never as system freezes. And
> last bios update stopped them 2 months ago).

FWIW, Andres Rodriguez reported similar symptoms with a Ryzen system on IRC,
and raising voltages / disabling Cool'n'Quiet / disabling C6 states fixed them
for him.