I don't think it is the shooter lane switch. I removed the glass on my game, started Crate multiball, removed the balls. When I manually triggered the shooter lane switch the auto shooter solenoid fired at me - the multiball remained active. So the game thought that an air ball had somehow found its was back to the shooter lane.
Sorry if obvious, but do you have the right number of balls in it ?
As a quick check, I would start a game. Remove glass. Catch the first ball. Then bang and fiddle around with the playfield to see if anything registers any points when it should not. Any hair trigger switches/ points accruing from nowhere could be relevant to the switch matrix issue below and could result in random errors.
Right software ? Did an owner put a dodgy ROM in it ? Did it work OK in the past then suddenly gave trouble ? Battery acid damage to MPU ?
Does the Crate multiball work properly while ball save is on ? If so it would suggest a trough problem.
I would then try to identify if this is a problem specific to two ball muliballs - or whether it is specific to the Crate 2 ball multiball only. The wizard mode and Coffin multiballs obviously involve more than two balls - so you can test this out.
Take glass off. Start the various multiballs and catch the balls and remove. Manually trigger all the switches to see if you can repeat the Crate multiball failure. You may find that the two ball multiball issue is common across all three of the game's multiballs. You may also find that some random playfield switch triggers the issue.
I reckon you might find that the problem also occurs when the wizard mode and Coffin multiball get down to two balls only.
You could also start Crate multiball. glass off. Grab the two balls and manually eject a third ball during ball save by operating the ball eject solenoid by hand. Does this three ball Crate multiball remain stable ? I tried this and it did remain stable on my game. Indeed the autoshooter fired the third ball into the game.
My first guess would be a ball trough problem - either bad opto, bad board, bad connectors. It sounds like you have been down this alley.
My second guess would then be a switch matrix fault only revealing itself when two balls are in play. So a switch elsewhere with a bad diode is giving a phantom result in the ball trough meaning when some switch somewhere is triggered the game is registering a drain.
If you put it in switch test, does every switch on the game behave as it should ? Remove glass. Manually remove 2x balls from the game and put into switch test. Manually test each switch and see if any trigger the ball trough as well. If you get the switch matrix graphic up and bang various areas of the playfield, do any switches have "hair triggers" ? or phantom triggers (2x switches register when 1x switch is triggered) indicating a blown diode on one of the switches ?
Have a look at this video, about how these diode problems form "squares' or "rectangles" in the switch matrix. If you find which switch triggers the phantom you can draw a square/ rectangle on the switch matrix and this will isolate your problem.
