In general, there are a few cheap fixes you can try.
The grease applied at the factory sometimes dries up or clots, so clean off all the old grease on the tracks and reapply some new stuff.
Also, the belt often wears and that can usually be replaced as well.
Finally, the motor can get weak and may need to be replaced.
Of course, sometimes the cause is a combination of the three. So the best course of action is to start with the first and second. If they don't do the trick, then you'll have to see about replacing the motor if the part is still available, or sourcing a donor unit.
The grease applied at the factory sometimes dries up or clots, so clean off all the old grease on the tracks and reapply some new stuff.
Also, the belt often wears and that can usually be replaced as well.
Finally, the motor can get weak and may need to be replaced.
Of course, sometimes the cause is a combination of the three. So the best course of action is to start with the first and second. If they don't do the trick, then you'll have to see about replacing the motor if the part is still available, or sourcing a donor unit.