The stepper units are a bit harder to fix. What you need to look for is when the coil that gets stuck on does anything on the stepper unit move? If nothing moves it might be siezed.  Try moving the unit by hand to see if it moves if it does then there may be a switch thats not being activated by the coil to tell it to stop energising. Which stepper is it? Is it in the backbox or under the playfield?