Today I received new motherboard. Spent 1.5 hours to installing it. And the symptoms are still there.
Also, this is a reminder why you should never buy Asus Prime boards, and only Asus ROG. The quality is atrocious, from the almost non existing heat sinks to the fact that you need to install back panel yourself.
If not motherboard, then what? I read some more. Seems some 5900X CPUs are faulty.
The problem? For RMA you need S/N, which is on the box, but I don’t keep boxes for CPUs, only for motherboard. So I had to remove cooler again to get S/N. And since I already started doing it, I replaced ROG motherboard back. Now it only took me 45 minutes. Improvement.
I disabled C-State. It helped, for now.