I know this sounds too simple, but last time we had that issue the tech sprayed ours heavily with WD40 after pulling the pin from the motor, worked the steps and cleared a ton of sand out of the joints, replaced the pin and everything worked. I now clean then on a regular basis. 

The other issue we've had was the little magnetic switch on the screen door which signals the steps to open and close. The magnet was out of place, moved back in position and all good.