More info.

I got a dummy hdmi plug.  So far my VEGA 64s have not needed one.  They always run fine monitor or not.

While xmr-stak-amd was mining I plugged a dummy plug into one of the two problematic cards.  Everything froze, screen went black, flickered a bit, then came back.  xmr-stak-amd looked stalled but eventually came back to life.

Check hash rates.  The one card I plugged the dummy plug into was hashing at 1900H but only for ~10sec!!  Then it dropped back down.

So, I figure lets reboot with the hdmi dummy plug connected from the start.  If I do this, I cannot enable HBCC on any card as the machine always crashes. 

So the best I can get now is to get everything all setup for 1900H (clock speeds, HBCC toggled).  Then once xmr-stak is running I can shove a dummy plug into a card hashing low and get ~10s of 1900H before it drops down to 1300H.