I need help with a finicky garage door opener. We have had the Craftsman 1/2 hp garage door opener for 4 years without problems. Now the door opens about a foot and then stops. I hit the button again and it closes completely (from car or house). I hit it again and it may go all the way up or get stuck again in the same spot. It always opens up by the third try, but is rare to go all the way up on the first try anymore. The sensors have been cleaned and tightened, and no lights flash when it gets stuck. Any ideas on what could be wrong? Thanks!