Me again ;)
So I found a Reddit thread which discusses a 68c temp limit for the 2990WX - had no idea this existed before building this machine
(
https://www.reddit.com/r/Amd/comments/9gidma/amd_precision_boost_overdrive_pbo_ramblings/)
As my CPU was hitting this AMD specified limit, the chip was failing and the motherboard had stopped communicating with it completely, I'm guessing some kind of fail-safe in the 2990WX causes this but it is not publicised anywhere and has driven me crazy for weeks!
I have now disabled every possible option for running this chip above the 3ghz base-clock, it has an auto feature which takes it up to the 250w limit but the temperature limit counteracts this and causes system failure.
Max wattage for the CPU is now capped at around 215w (250w limit) and the benefit is that my temperatures have yet to exceed 60c - and guess what, my animation is chugging along nicely.
Don't let any of these issues put you off building a machine with this chip - when it works it really is unbeatable value for money. Just be aware that there are a ton of built-in functions which try to push this chip to increase speeds, but then those same built-in functions have to fight against the temperature limit of this massive chip, and that's where the failures begin to happen.
Unless I can work out how to A) increase my total wattage allowed to be used by the CPU above 250w, and B) keep my temperatures below 68c (apparently this isn't currently possible due to AMD's own overprotective restraints) then I will have to use this chip at it's lowest and slowest speed of 3ghz.
So happy I can now leave the room and let my animation fly knowing I won't come back to black screens and an unresponsive box of heat!
PS - also had to lower RAM speeds from 2666mhz to 2133mhz - again resulting in slower rendering but at least it works