Well, if your gpu works in another pc and your replacement board is acting the same it's sure looking like it's the cpu, I can't see you getting two boards with the same problem.
I don't know if ram instability would cause that, you cleared the cmos which resets the bios to defaults so it's likely not the ram.
You could try reseating the gpu and make sure it's fully seated and clicked into place with the pcie clip. Double check the pcie power cables are fully connected into place.
It's strange it happened all of a sudden after a bios update and now same behavior with the second board. If the psu is 100% working then I'm staring pretty hard at the cpu. You could try flashing the bios to see if it would help, it sounds like you know what you're doing if you're able to enter the bios after updating it.
Is the motherboard giving an unusual code ?