I have the same thing; I believe it is a fairly common issue with the microswitch in the door lock failing, so that the door thinks it is closed all the time, even when it is open. So when you lift the handle to open the door the window drops, but then as soon as you release the handle, the window goes back up.
This article talks about the assembly to be replaced:
Pelican Technical Article: Boxster Door Lock Mechanism / Boxster Door Handle - 986 / 987
If you are a member of Renntech.org there is an article that goes into more detail about which behaviors mao to which microswitches:
http://www.renntech.org/forums/tutorials/article/217-door-microswitches/
It's in the 996 section but it is the same.
There are a couple of folks that have been able to replace/replace just the microswitch and not the whole lock assembly. That is what I hope to do when I get around to taking my door panel off. Luckily mine is on the passenger side and so it is not as big of an issue as it would be if it was on the driver's side.
Steve