Possibly fixed ish.
Tried a few other things as convinced its more of a GPU / monitor / switching issue , swapped dp cables, ports etc still same. Monitor is a Benq XL2730Z, had a couple of years without issue. Using the Benq monitor driver / 144 hz.
Noticed stuff in Radeon Settings / Display / Specs / Overide
Googled "Adjust Voltage Swing" and found this mention by AmdMatt. ( Searched in the AMD driver thread here and didnt find it mentioned )
https://community.amd.com/thread/212160
Changed Adjust Voltage Swing from 0 to 1 and the problem seems to have gone away. Switches in and out of tests fine during Firestrike. Put back to zero and reboot, issue comes back.
Wonder if boosting power to 25% / 50% in Wattman somehow achieves something similar, i.e. stops voltage somewhere along the way dropping a bit too far for the output signal.