First thing to check is to make sure the window glass runs are not damaged or dirty. If the window binds in the glass run it will be jerky. Basically the motors either work, or stop working at the same spot each time if the motor is bad. The diaphragms are a type of soft plastic /rubber material. Don't try to spray anything inside the motors. Oil will just settle to the bottom of the motor and be ineffective. I repaired all 4 of my window motors on my 1951 Manhattan. Now they go up and down faster than electric windows. The speed can be adjusted and just before the window goes all the way up, the window slows down to close softly. That is the cool feature. Also, on the pump, there is a fine screen at the vacuum inlet that can be partially clogged. Just clean that with some compressed air.
