Playing and searching around a bit with it, and I think it is an issue with the solenoid expander relay. Both the outhole ejector and the knocker have the same output wire, so I think that if the relay is not in the right position, the knocker will fire instead of the outhole ejector.
(This is where I should mention that this is all new to me. I just did some reading up and looking at schematics. So, if I misstate something or say something completely wrong, have mercy.)
The relay has a 555 bulb powering it. I read about a test where you put the pin in all lamp test mode. The lamp should blink on and off and the relay should click. My lamp seemed to stay on, and the relay only clicked some of the time. Other times, it was silent or perhaps had a very faint buzz.
I replaced the 555 bulb just in case and started a new game. The same thing happened when the game tried to kick out the ball into the shooter. Finally, after a few tries, it kicked out the ball correctly, and the game played perfectly. I tried several games and it played well. I then decided to do the all lamp test again. Same results as before. I then started a new game, and the ball would not kick out -- just the knocker again. Once the ball finally kicked out after several tries, the game played ok. I did the lamp test one final time, and this time had all clicks, no silence or buzz. I started a game, and it played well.
I hope this makes sense. I am guessing if you are familiar with solenoid expander relays on old Ballys, you will know what I am talking about.
At this point, I guess I should just watch the game and see if it continues to recur. Assuming it does, what are my next steps? I read an article that sounds like it could be any number of problems with the relay. Is there any way to figure this out and narrow down the possibilities?