Not a software problem. Sounds like a trough opto problem. Make sure they're tight. Both boards on my Magneto were so loose,they were flopping around. I got a new set from Stern,but ended up not having to use them because of this. Tightened them up,& no more thinking the ball has drained.

Sounds like a problem with the ball trough optos, yeah. Haven't worked on ac/dc but it happens to every stern that uses them and some DE/segas too.

Empty the trough and make sure there isn't any debris in there. Make sure the connector to the opto boards is on the pins tight. Make sure the receiver and the transmitter are pointing at each other in switch test or empty ball test.

If you didn't find any problems there take the opto boards off and reflow solder to the pin pads. If that doesn't work, you may have to replace the opto reciever and transmitter or replace the boards.

In my experience, and as others have stated, the opto boards in the trough are the culprits. Makes sure the pin connectors for both opto boards are firmly on. Shaker motors and heavy nudging tends to shake those connectors a hair off the pin which will cause this issue.

I had a similar problem on my game. It turned out to be a broken ground wire on the connector just before the optos. The wire was hanging by a thread, once I found it and removed the connector, it snapped right off. Once the wire was fixed there were no more problems.