We had the same symptoms and finally replaced the motor with one from a GM electric window application. $39 and it works better than the OEM one ever did.
Lee and Anne
2016 F250 2WD CC SB XLT 6.2 3.73 3,295 lb payload cap.
2014 Salem Hemisphere 282RK 8k lbs loaded, Equal-i-zer WDH, TST 507 TPMS
2016: 33 nights camped; 5 nights booked