Ok - so i've narrowed it down to a kworker process which is hogging all of a single core and causing the system to slow to a crawl and eventually freeze.
its alternating between kworker/3:7 or 3:10 or 3:6 or 3:9 or something similar.

does anyone know the best way to investigate what these kernel tasks are trying to do which is causing the system to slow so much?

screenshot of htop: https://ibb.co/g0FCOw

EDIT - this ONLY happens when i run 12+ GPU's this does not happen with 11 or less GPU's and i have changed risers/ swapped GPU's with no change.