All solenoids die or just the flippers?

Also is it resetting fully, like going back to booting and attract mode, or does it go to the next ball?

You sure you have the correct amount of balls in the trough? Should only have 5 in the trough.

Have you tested the trough optos in the switch edge test?

Does anything else happen? Like does it act like it's tilting? I think this is late enough to have a door high-power interrupt switch, is that functioning properly?

So if I understand correctly you are saying that at times the flippers die and it goes to the next ball but when it does, the flippers work fine although they may or may not quit again on the next ball prematurely? If that’s the case, then I think there’s probably something telling the game the ball’s over versus something with the flippers. I know on my Judge Dredd it did what you were talking about and it was the trough optos.

Since the flippers die and its going to the next ball it in fact sounds like it thinks balls have drained and actually isn't resetting. When a ball drains it is a common setting for the flippers not work to save wear on the flippers in a route location situation. Does it count your bonus when the flippers die? If so I suspect you have something flakey with the ball trough opto boards. If you have a set you can pull from another game, yours or a friend's, swap them in. You can always pull the board with the infra red leds and look at them with the game on through a digital camera. All of them should be lit. However the receiver board you will have to use your multimeter to test it. I bet its the opto boards. You can always try reflowing solder on the connections on the opto boards. I've fixed a few this way.

Transmitter optos do dim over time before they fail completely. You may even see the difference in brightness with a digital camera/cell phone camera. When I get a game and have problems such as this, I change all the transmitter optos in the game and just clean the receiver optos (rarely fail).