See this thread for a potential solution:
http://986forum.com/forums/general-discussions/25704-window-regulator-permanent-fix-stretched-cable-3.html
If you find that your window has a bunch of free play, meaning that you can push the glass up and down about an inch or more with your hand when the door is open, this could be your problem. The good news is that if it is just too much slack in your regulator cable, you can fix it in less than an hour for less than $3.