Quoted from Boslaw:
but then randomly on the 4th frame (or the 6th or 7th or 8th) the frame unit will skip back to 1 or some other earlier frame. It's random.
Shooting from the hip here but could the Frame stepper be getting a brief reset pulse during the game, or maybe a reset pulse at the same time as an advance pulse? You could put a bulb tester across the reset solenoid solder lugs to see if it ever fires when you see the problem.