actually this is not really a common issue at all but in some locations avaialibility of junk pathys is so low that when you find one there isn't much left. edit: OR should have a bunch though unless 88 beat you to it all, although he isn't in the S.
if you purchase a new or newer motor make sure to match the old one with the new one... they made a couple of different 'models'. at least the way the connect so, don't get stuck with one that won't work for you. one had 3 prongs that go into the door's drive and i don't recall what the other one was
oh, the gear thing.. if you say the teeth look perfect then the cause could be latteral slipage, as the cog moves side to side enough where it doesn't line up with the other gear. i have a similar issue with a plastic gear in another car on another part but that was what i finally figured out; it may be something to observe.