I have a genie garage door opener and it will bring the door all the way up, but when its time to go down, it goes about 8" then back to the top. I've disconnected the sensors thinking thats what that was. If you hold the sensor that tells it that its gone up far enough, and push the button for it to go down... it will go down. If you hold the garage door opener button down... it will go down, but not a momentary one push. Anybody have any ideas on what could be going wrong?