This might be a shot in the dark, but I'm wondering if the laminations on the commentator need a clean. This link shows how to do it.
http://vssmb.blogspot.com/2011/12/ho...r-part-17.html
I know its not a potted motor, but I think the commentator and carbon brushes are the same.
Edited to add, go down to the short video when he is cleaning the laminations with a drill and sandpaper. Its quick and easy and not too much fuss.