Had this problem a few months ago. Window had been acting up for a time- it was not going up evenly and weather stripping was getting caught. Then, it tried to go up pretty much diagonally one day and that was my clue to take it apart and see what happened. I figured it was a problem with the rear track and that's what it was. Rear track is fastened to the door. Well, the tab broke off and the track fell to the bottom of the door. Solution was to reattach the track to the door. I used some bendable pipe hanger iron (it comes in a small roll and has holes in it) and reattached it to the door. Still works fine.