On the 5950x, try disabling C states (not P, C; the ones where it 'sleeps' CPU cores). And verify they're disabled within the OS.

It took most of a year to nail that one down.

AMD didn't ask for anything besides proof of ownership once I told them disabling C states fixed it. And, 5950x's have a 5 year warranty...

PS: if they were Northwood P4's, see: https://www.overclockers.com/forums/threads/the-official-sud... (I didn't name it. I agree, terribly insensitive name)