The other day I had to tear my fan apart becuase it wasn't operating...at all. I figured it may just need some brushes or something since it still spun smooth by hand and all. Turns out, one of the bushes got stuck in its housing a LONG tim ago. So I have nearly mint brushes on a perfect commutator and am good to go now. BTW the bearings are probably going out on your fan..if I recall, they are ball bearings...front and back. If you can find a used fan that spins good or something you may want to do that...they are sorta weird bearings and probably not worth the time if another unit is available.
Best of luck.