I had a similar problem and the fix was a new door lock mechanism as described in this thread:
http://986forum.com/forums/performance-technical-chat/15088-door-window-drops-but-immediately-jumps-back-up.html
My window would drop when I pulled the handle but it would not stay down until the door was closed. It would jump back up as soon as I released the handle. The door lock mechanism solved the problem. This feature is controlled by microswitches inside the handle.