@DeepT Can you confirm you checked this as well? @lordkosc posted about it, I posted about it saying it was a good idea, and now @saracen31 has as well. It’s by no means a slam dunk but it’s super easy to rule out and it would explain everything you are seeing.
One thing I want to make sure you understand is you only ever change the maximum mulitiplier - you don’t change the base 8x. It’s very normal for your machine to be sitting at 800mhz if it isn’t doing anything, but it should immediately ratchet up to 5ghz if it starts to do something. Just running Prime95 for a second for example should make it hit 5ghz - which I understand you have done, I just wanted to be clear on the rest. Some of your wording above made me suspicious =)
Loading Optimized defaults and touching nothing else should make it all work at 5ghz unless there is some specific issue with that board and cpu that we are not aware of. I would say it’s never the CPU for an issue like this but at this point it’s getting weird so it very well may be.