Dave- sometimes it's not the motor( it still may have to be opened up and old lube cleaned out and reapplied)
most times it is the window-binding up on the sliders, misalignment in the door or even the switched/wiring/ground or power source not suppling enough power. using it with the car not running off the battery- it will be slow!!
if the car is running( off idle to get the alt to put out juice),and trying it will give the motor more power to operate properly. so if the motor is trying to work- I would suspect the window is binding somewhere.
John Mac
check these
