Most likely a stepper issue. I am not familiar with this machine but if I had to guess I would be looking at the ball count & player units. Making sure that they are moving through there proper motions without hang up. This also means inspecting the switch stacks associated with each unit and making sure they are engaging properly and in the proper state.

Again this is a blind suggestion.

Check coin unit too. And make sure your stepper contacts are clean and aligned on the center of the rivots. Not touching two simultaneously.