The other day mine was doing the same...one of the switches in the subway had a wire that was no longer attached...i had to solder it back on and it started working again...not sure if that is something you want to look at...there is also an arm inside the subway that holds the balls in place and releases them 1 by 1...that can break and need replaced...if the balls are moving past it, the trap door will open and shoot a ball onto the PF...not sure of the part # for that arm, but there was another post about here recently...same issue you were having

Daniel

There is an opto in the trunk itself that senses when a ball falls into it. Does that switch work? That's what does the "falling down the stairs" animation outside of true lock mode.

There are also switches in the trough/subway. Doe those work? Use a pinball and not your finger to test switches.

Marc

I know this is an old post but I was having the same problem, found this thread via a search and thought I would let folks what fixed it for me. I admit I'm pretty much a newb so I apologize if this is old info for most.

For me I was changing some lights over to LED and after doing so I started experiencing the same multi-ball issue. It turned out that one of the micro-switches in the subway was hitting a wire that I must have bumped. When it was got in contact with that wire it prevented the microswitch from closing once it got opened. That caused the issue. Once I moved the wire out of the way and the switch worked again the multi-ball problem was resolved. I hope your solution is as easy as that.

Steve