I'd first manually operate the player unit to see if it turns smoothly and clicks into each position. The grease they used on the unit at the factory can turn into "goo" over time and the unit will have problems operating correctly. Sometimes the unit needs to be taken apart and thoroughly cleaned.

Do you have a schematic for the machine? Does the problem always happen when you set up a 1 player game, 2 player game, 3 player game and 4 player game? When you said "one player," was the game set up for a 2 player game?

If you have the machine set up for a 2 player game, and the machine will switch to player 4, then I would look more closely at the coin unit. When this issue happens, what is the position of the coin unit wiper switches. Is the coin unit resetting when the machine is reset?

You can apply a very thin layer of Teflon Gel Lube to the disc contact rivets. Only use a very limited amount of the lube on each rivet.

Also, check out Clay's Pin Repair Web site about care and maintenance of stepper units:

http://www.pinrepair.com/em/index2.htm#steps