I would check out your driver board also. You should be able to dial that voltage down well below 190. Not going to solve your issue at hand, but may save you later.

When I rebuild these old driver board I dial the voltage down to like 175. I know folks go lower but that is going to bring up the amperage on the circuit as the displays continue to draw for power.

As for your display issue as me as a plus one for connectors. I would replace the .156 headers on the displays. I would also repin the female side with Trifurcon style connectors. If you grab the connector with the game one and twist it slightly dose it act up? Be careful as there is 190v there.

You have taken the same trouble shooting steps I would? Not to say that is all of them. I still would change the male header before I would go crazy. Dose it stay with the same digits or move around?

As for the driver board. I would look deeper into the circuit. My guess is you have a resistor out of spec. The pot is doing it job. Opinions vary but I just change the all the headsets rebuild the 190 and 5V and do mods to those boards pretty much right after I get a new game going. The 5V is critical to the game operating and bad 190v can damage displays.