Thanks for the links, sounds like there’s a strong correlation to HDMI expansion cards. I totally missed that full HDMI isn’t supported at all in slot 2, which is exactly where I stuck it 
. I’m re-shuffling my cards (I also had USB A in a “high power consumption” slot – I guess I really wasn’t paying attention to the compat matrix during setup 
).
I’ll run this for a while and report back with results!
(Note to self: If this doesn’t do the trick, try disabling “PCIE Dynamic Link Power Management”)