Check the mech. - see if it's sticking. Manually activate the advance and reset coils (with the game off). Most of the EM problems just require cleaning, and perhaps a bit of adjustment.
If you reset it to the player 1 position, and start a 4 player game - does it advance through the players correctly, or keep hanging up? If you manually reset it and it just slowly slides back to the home/player 1 position, it's probably gunked up, assuming a spring didn't pop loose/break.
Even without the schematics or manual, you can carefully pull it apart and clean the linkages - just take pictures first. Review them, and make sure you have everything you need to know how it goes back together.