I don't know your mechanical/troubleshooting ability, but try this. Disconnect the door from the trolley and operate the opener. This will show how the opener is operating without the wieght of the door. If the opener works without the wieght of the door and the door operates smoothly by hand, as you mentioned, the load of the wieght of the door is bringing out the problem. Get on a ladder to observe the head unit and push the remote button and watch. It may be helpful to remove the outer casing of the unit to get a better view of the moving parts. Other than strech in the chain, which can be adjusted out, most problems are in the head unit. These are very simple mechanisms but often contain cheap plastic or low quality gears. I recently had a problem with the door at my parent's house that sounds similar to yours. It turned out to be a worn bearing that was easily replaced. Look to make sure the shafts are tight and rotating without play. Also look for metal powder or dust that could be coming from worn components. If you still have the owners manual, look at the exploded parts diagram. This can help.