I suggest 2 things.....
visit this website and do a search for "slide motor"
You will read some other folks woes and fixes.
They ain't cheap anywhere.
If your motor is acting up but the plastic gears are still OK,
you might try taking it to a local repair shop that specializes in
motors and or automotive starters/alternators.
If they are feeling helpful they might be able to coax some more life
out of it.