My wife's Audi was doing something similar, the driver's side window was getting slower and slower and eventually stopped moving at all. I thought the motor was fried and I as not liking the idea of replacing it (for the tune of several hundred dollars for the part). After removing the door panel and unbolting the motor from the regulator, I plugged the wires back into the motor and tried the switch only to be pleasantly surprised by a working window motor. Long story ---> short, the window regulator was binding up from a lack of lubrication. (This car happens to have a cable-lift style regulator). I sprayed WD-40 over everything and used clear silicone grease where the glass rides in the window channels. The window has been quite happy ever since.
Hope this helps,
Does it always get stuck at the same position?