On a Gottlieb 4 player the player unit activates the ball the in play. It's operation is somewhat controlled from the coin unit. The coin unit controls the amount of pulses the player unit gets to achieve the next ball in play depending on how many players are coined up. Not sure if Sonics work about the same. Your player unit looked like it may have been gummed up and sluggish. I think your player unit is going to activate a switch on your score motor to activate the ball return solenoid. If the player unit isn't functioning correctly or the switch on the score motor isn't gapped properly then that may be your issue. If it was me though and since it may be going to the show where it would be nice if it continued to run, I would go through everything, cleaning and gapping along the way. Sounds like there may be many little issues. The levers on the player unit and every other unit need to be free moving. The springs aren't strong enough keep them moving. Sometimes people grease them up and after many years that turns to crud and sticks it all up.