I have a Craftsman 41A4315-7D garage door opener. All of a sudden it won't stay closed. The bottom of the door gets to within 12" - 18" from the floor, then suddenly reverses direction and goes back up. The light flashes about 8 0r 10 times, but it will stay up. It just won't go all the way down and stay down. I've check and double-checked to make sure there are no obstructions. Any ides?