I think that era has a reset bank towards the front of the bottom of the cabinet. Should have a PB relay (2nd player relay) that might have something going on with it...not sure how those work, later games had a player unit that would need cleaning. Someone with a schematic might be able to tell you what controls 2nd player relay.

Also make sure the stacks are tight on the player unit, this can cause similar flakyness.

Ive also had an issue where the throw of the plunger into the coil of the player unit was a little long causing it to sometimes jump two steps. This is fixed by loosening the mounts that hold the coil assy, in place, you'll see slots allowing up to 1/2 inch etc..

Manually activating the plunger and watching how much cog/gear it graps will give you an idea if this needs shortening.

Ken

n!teowl
I agree. Sounds like the stepper unit. I cleaned the contacts with Barasso metal cleaner and it works great. Did you also clean up the spring loaded contacts? Check the wires connected to those contacts also. You can also try fine tuning the position. It might just be falling off the contact when it gets to that position. Good luck.