My drivers side window did this approximately 1 month after my car was delivered (European Delivery). It worked fine when the car was moving or if the throttle was open, but if I was in a parking spot, it would come about half way up then drop down again.
I took it to the dealer on my first service interval finally and they said it was a motor issue. The stock motors have a capacitor problem in that at idle, the motor does not get the full 14.4 volts it needs to function properly and thus the software program does not run the window properly. It isn't a problem when the engine and thus the alternator is turning which is why it worked when moving.
Either way, the motor was replaced about 6 weeks ago and now works fine. No other issues with taking the door apart either which I was worried about. I suggest getting the motor replaced as it seems to work not only in my case but in others as well.