I just came across this and would like to say I have exactly the same set up as you and experienced exactly the same thing with my deteriorating 5900x.
The sad bit is I bought it from my local shop and lost my receipt. AMD is refusing to take on my case because of this. I believe they are now making RMA as hard as they can be because they know they have a faulty product and is now putting that responsibility on us consumers and completely turning a blind eye to it.
I am now going to everywhere I can to spread the news regarding this faulty processor that they are selling and absolving themselves of any of the predicaments that us consumers are suffering because of their ineptness.
Just like how this thread has been treated with absolute silence from the AMD support team here.