One core on my 3960 became unstable anything beyond 4ghz, basically I turned off Core Performance Boost so none of the cores would boost and it would boot up just fine, run Windows etc. Yours sound very similar to my experience, same exact symptoms, mine was CPU.

Some things I did:

Then I manually OC to see if I could find a point where it failed and that was around 4ghzI then manually OC one CCX at a time to over 4ghz, something like 4.2ghz and found the core causing the problemI then OC all the other CCXs, keeping 1 CCX slowPerformance was affected weirdly, depending upon Windows scheduling, it could use the much slower core and basically tank single thread stuffI RMA it to AMD indicating my findings on which core went bad and they replaced it. They sent a new in box replacement which performs better than the first 3960.

Reason why it can boot into bios is most likely is that it is not boosting to failed point.

Why the first 3960x failed, I did not use PBO for any length of time except for testing, only using Ryzen Master, never in the bios until the core failed. Used Ryzen Auto OCing once again for testing but never used it beyond that. Since these CPUs are the bad of the bad, higher power, one less core per chiplet, there maybe more prone to failure and really I do not know. I have a thread here going through this, too lazy to find.