Updating this to say it's been a few weeks with no official AMD response to me after providing this information.
I have tested the grounding in my home, and they are all good.
Same issue. done everything aswell as you OP, still have extremely high fps and ocassional 1/8 to 1/4 a sec stutter without any varible besides the cpu, if i change the an intel CPU it actually still occurs. Looking like OS issue with windows 10.
I have been having the same problem you have too so i sympathize with you completely, your post should be pinned so other know that nothing is solving the issue, i have thought about getting an intel cpu with a new mother board too but the only thing stopping me is that i have seen the same issues happening to people with intel cpus too, so i am really at a loss at this moment
I am currently in contact with AMD.
I was asked to create a fresh install on Windows 1809, latest Nvidia
drivers and chipset drivers and that is it. Then proceed to download a game
and test it.
I had done this in the past, and experienced them. Once again, I have
experienced them again, but not quite as bad.
I did notice an increase in FPS for some reason. But the stability is more
or less the same. Still stuttering.
AMD are attempting to recreate the same setup that I have to try and
recreate the issue.
Something tells me that they won't be able to.