I have a similar problem with driver side power window. The motor is fine. It raises the window part way. The motor continues to run but the window doesn't move unless I lift it. Then it continues up until it does the same thing again. Sounds like a busted gear, right? I removed the regulator and motor and disassembled. The gear(s) are fine - no chips, broken teeth, etc.
I suspect it may have something to do excessive play or adjustment with the worm gear from the motor. I see what appears to be an adjustment screw with a locknut on the end of drive shaft.
Anyone have any idea on the "concept" of that adjustment? (ie. Adjust in to get what result, adjust out to get what result?)
Btw, it all works fine when I bench test it. It only fails when I have it installed with a load on it.
Thanks.