Connectors, connectors....

Can you post a picture of the board? Use an ohm meter and check all your resistors for proper value, then use the diode mode and check your transistors.

Displays look to be in decent condition. Headers need to be cleaned up, one on the left needs reflowed and the one on the right has too much solder. Remove as much as the old solder as possible then reflow them. Then clean up the flux with some 90+% isoprople alcohol. If the resistors check within tolerance, especially the 100k (brown/Black/yellow ones) then about all that is left are transistors. Even when they check good in diode mode they could be breaking down under load. Do the same digits light up when it is acting up? We are taking digits here and not segments, correct? If the same digits are acting up then it is probably the digit driver/level shifter transistors. I typically change them both at the same time. Take a look at Steve Kupla’s page, http://stevekulpa.net/pinball/bally_disp6r.htm , for some more hints on troubleshooting these displays. They are not hard to fix once you get and understanding of how they work. Look at the schematic here, https://www.pinitech.com/retrofit/schematics/bally_as2518_58_7digit.pdf , if it doesn’t make sense ask questions.

^^ This is important.

One thing you DON'T want to do with these is flow new solder over the old. The old solder is crap and simply flowing new solder over the top will result in a crappy connection.

If you're going to remove the old solder then why not replaced the old header?

If I were working on the display I would certainly replace the header because I have the .156” headers and a Hako desoldering gun so it takes me about two minutes to do with zero damage to the board, others may struggle with it, I know I did until I got my Hako. I also replace all the 100k resistors with 1/2 watt ones.

One other thing to check that I forgot to mention above is the high voltage. Make sure it is set around 170 volts.

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.

Did you run the self test on the displays?

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.

I was having a similar problem. In test mode it would count 1-7 just fine then when it displayed 8 and 9 it would show random numbers. I thought bad decoder. I replaced the header and connectors and was about to replace the decoder but first i tried it again. The new header fixed the problem.

My Fireball Classic was doing this and I found out the mpu was not making good ground.

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.