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,