It could be the limits have been damaged or are jammed and are jumping on the screw adjusters. Your could take the cover off the motor to have a look to see if you can see any physical damage. Usually if you press screwdriver into the adjuster to hard it can bend the limits.
Other than that i would suggest contacting Andrew @ Liftmaster Technical Helpline on 01935 823900 to check see if he has a soloution.
Other than that i would suggest contacting Andrew @ Liftmaster Technical Helpline on 01935 823900 to check see if he has a soloution.