Quoted from chuckwurt:
Put the game into switch test while you’re doing this and see if you can see any odd switch behavior.
Good suggestion. Make sure all the switches register correctly in switch test mode and there are no switches that are unexpectedly triggering other switches. Pound around on the playfield to make sure vibrations aren't triggering anything.
Still, it sounds like it's not the switches. But if you wanted to be 1000% certain, you could just set them so they are completely open or completely closed at all times and see if the problem persists. In either case, you would not see this behavior on a modern Stern. (Open at all times would be better because a closed switch might actually be opening and closing if the contacts are bad).
(But it sounds like this has been tested. You can see the switch is still open, even when the problem is happening. Please be *certain* that this is the case, because what you're experiencing is exactly what could happen if the EOS gap was too small).
If it were me I would just replace the transistors because that was the only thing that was changed. But that sucks if you have to send away the board to get it done.