So I made one last attempt (I'm out of thermal paste!), this time with the HD5770:
- boot seems normal with Apple logo and progress bar
- right before getting to the login screen I get a KP
- forcing the computer to shut down is the only option
I shut down. Reboot with NVRAM/PRAM reset. Always the same.
So I'm back, again, to the original 2.4 CPUs, and the RX 560 (at least this one works!).
Like I said, I'm now out of thermal paste... I had just enough for one CPU and barely for the second one. Everything looks normal though on the temp side... So I'm just wondering right now if I should try with another pair or just leave it at that. There are several options on ebay around $100 a pair (5680). That is so, sooo frustrating!