I have a Genie IS550-2 that is about 10 years old now. Recently, the door sometimes will open 1 foot, then stop. Next time, it may open 3 or 4 feet and stop. Sometimes, it works perfectly. Closing is the same way. I have readjusted open and close limits and force but it makes no difference. I removed the carriage and it is not stripped. The screwdrive also looks good. Any ideas? Thanks, Rick