Have you checked the mechanical action of the stepper unit? I have spent many hours chasing ghosts in electrical circuits only to find the catches, pawls or ratchets need lubrication. It has been awhile and I don't have a game to look at so excuse my poor memory. I don't recall if its the player unit or coin unit that advances ball count. I seem to think it was in the bottom cabinet near the back somewhere. With the power off try moving all parts in stepper mechanism by hand. Pay close attention to the catches on the gears and other locations of the unit. If they are hard to move shoot a "little bit" of WD-40 on the axle and work the catch until it moves freely. Do this for all suspect steppers. If this solves your problem you can do more intensive cleaning and proper lubrication on the suspected parts. Good luck!