Ok so here's the poop:

A friend calls me up to look at his EBD because his left flipper stopped working suddenly. When you press the left flipper button, nothing happens but if you manually actuate the flipper and hold the button, it holds. Clearly an EOS issue so I fixed that and everything works fine. Since I am there, I decide to clean and wax his machine, change his old rubber and a few burned bulbs (no LEDs on this one).

Anyway, I button the game up and go to give it a test play...and the coils are all dead. No flippers, no outhole kicker, no drop target bank reset, nothing. I turn the game off and check all the fuses as good, each test under the playfield and in the lower cabinet. I open the backbox and check the fuse on the driver board, also good. Next I use a test wire and ground some of the transistors to see if the coils fire and indeed they do.

I turn the game off and back on, press the start button and the game fires up like normal. Everything working! I'm confused as to why its working all of a sudden. So I play a test game and everything is fine. Turn the game off and back on, try again. Nothing works. Same as before, game starts but coils all dead. Since I had the backglass out, I grounded a transistor again and again, it fired testing good, and immediately all the coils start working again and the ball kicks into the shooter lane. I play another quick game and turn it off and back on. Start a game, coils dead, same as before. Just like before I ground a random transistor on the driver board, the coil fires and the game works normal immediately after.

At this point I was out of time and really had to leave so I told him I would think on it and see what I could figure out. So I closed it up again, I restarted the game and tested again, expecting the coils to be dead again but to my surprise it started a game and kept working (without me having to ground any transistors). He kept the game on the rest of the night so he could play and it continued to work fine. The next morning he texted me to say that he turned it off that night and in the morning the coils were not working again. So, I need to get back there to troubleshoot it again soon but I really don't know where to start. Any thoughts or suggestions would be much appreciated.

The MPU is an Alltek Ultimate MPU board so its connectors are good. I have already re-flowed the solder on all the connectors on the driver board. It is an older Stern board and had several cold solder joints so I did the whole board. This was before fixing the initial flipper issue.

If it was a connector issue, wouldn't my manual grounding of the transistors not caused the coils to fire? I could pull the driver board again and double check my work but I feel like that's not it. I dunno. I will probably be "shot-gunning" this issue and trying everything anyway. Thank you for the suggestion.

I did check the coils for power when they were not working and they did show about 43/44v DC at the lugs.

Okay! Thanks for the great suggestions here. I will be out working on this game again today or tomorrow and will report back with findings. I really appreciate all the advice and knowledge. Standby for the update.