As Mikala said, start with the Ball Count Unit. The steppers on Williams EMs are notorious for getting gummy and not wanting to step up and down anymore. By the way, if you start a two player game, does it play ball one for both players, or does it just stop responding after player 1, ball 1 goes out?

You can improve the speed with a thorough cleaning, waxing, some new coil sleeves, some new plungers and links, and any other obvious replacement part like springs, coil stops, undersized EoS switch wires, etc.