Sorry to reignite an old thread but after months pulling my hair out over this, I finally found the solution, so I thought I’d post it here (mostly for my own reference).
I found the problems were reduced somewhat once I down clocked my RAM (to 1833Mhz) but I was still getting freezes when both GPUs were at load. That led me to suspect it was a Ryzen/Motherboard thing not a GPU issue.
I changed the BIOS settings below and now I have a fully stable system under GPU load:
Advanced > AMD CBS > Zen Common Options > Global C-state Control set to “disabled”
Extreme Tweaker > Core Performance Boost set to Off
My suspicion is that the automatic Core Performance Boost was causing the instability.