Sometimes the windows lose their memory settings of where they should stop and start. You may need a regulator, but resetting the windows would be a good first thing to do and is free.
While inside the car and the door completely closed, run the window up and down a few times.
If this doesn't solve the problem, I'm afraid you have a worn regulator and it needs replacement. Our cars have this problem more than 911's of the same year because we have drop tops and we're constantly using the windows where a hard top driver might not roll the windows down nearly as much.
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