Ureka! don_1 was on the right track with the "black plastic interrupter cup". This is the part that is almost touching the circuit board at the back of the GDO on the end of the motor shaft. Unplug the unit so you don't touch the circuit board, like I did. I unscrewed the back panel and taped it up, out of the way. When you put the washers and the metal ring on the end of the motor shaft (1) make sure the first ring, copper color, with the 1 notch fits into the notch on the plate, so it won't turn. (2) Put on the other washers, make sure the motor shaft is extended as far as it will go, hand turn the worm screw. My problem was, the shaft will have small burrs from the old lock ring with the 2 small allen wrench screws so I could not get it on far enough. Take the old worm screw and a hammer and use this to tap the metal ring on farther. Tighten 2 screws. Now the black plastic interrupter cup will fit onto the end of the shaft all the way. That should do it. PS. Untape the motherboard so the wires don't get caught in the gears on top of the opener. I hope this works for you.

Sometimes the insulation on the wires running to the wall control are cut by the staples or the wall control shorts and can cause this problem. You can eliminate both by removing the wires to the wall control at the motor unit. Remember which one goes where. One has a red tracer. If this doesn't cure the problem you may have a bad logic board.

On the rear of your motor unit near where all those white wires go you will find two controls. One is the 'up force' and the other is the 'down force'. Turn the 'up force' about a 1/4 turn clockwise. This control will compensate for springs that are weakening due to age.

Make sure your door sensors have not been bumped and that they are pointed at one another. Each one has a small light that must be on and not flickering for them to work. If you cannot get both lights to come on you may have a bad sensor or wiring leading to them.