I have just moved into a house were the garage door had some minor damage to the top panel so the opener wasn't being used. I repaired the panel and found going up is very smooth, but as it goes down the door starts jerking. not violently but not a smooth operation. Is this due to an adjustment some were? I have no manual or other instructions. this unit is a chain drive. I fear this could result in more damage to the panel if not corrected.

any suggestions?