A clean garage door is a happy garage door. I take it that yours is chain operated. My Genies are screwdrive and will only work in cold weather when lubricated with clean "Genie" grease. (lubriplate white grease). If they get dirty or dry the safety mechanism will stop and reverse the door, thinking that it has hit something due to the increased resistence. Clean them up and lube them, rollers and all. If that fails to do the trick then call an expert or replace them. A faulty operner can cause injury and damage, not to mention being just darn irritating.