have you checked your window to see if its slightly off the track ? normally the "window dance" is caused by the window not sitting on the track properly or off the track. to check you need to take the door skin off and unclip your speakers, and then take a good look at where the window is connected there should usually be bumper stops on the end, and the rounded end clips should be connected to the channel guides.
on my 325is the drivers side starting doing same same thing about 6mo ago. Had it fixed at the dealer and they said it was a problem with the "anti trap" feature in the regulator. Whatever the trap feature is!! Now the fuc*&@ pass side is doing the same thing! I'm going to live with it for a while, due to $$$
still think that the plasic knob plug is not completely connected to the rail guide. i had the exact same problem , but it would only do it once in a while , then i fixed the knob that wasnt fully secure and ta-da it was fixed ..untill a year later it happened again... if you find this to be your problem use a good large set of clamps to pop it back in
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