Hope you figured it out by now....I didn't (sounded like the same problem).....Had Sears come out after I replaced plastic gears. Door would open 12 inches then back up same!
My problem was a broken "speed indictor" (small circuit board next to the spining plastic slotted end-cap. Replacing board is a snap....part only cost 20 bucks....final Sears service call costs....$120...ouch! (could of almost bought whole new door.....Sears not much help on phone!!