I'm sure this has been covered somewhere, and you've probably tried it, but my .02 would be that it sounds like a vaccum leak. Porsches are weird about that, in that most of the time they "cope" and still run somewhat normal unless its a big leak.
Get a handheld propane tank (small torch, open the valve, but don't light it) and wave it slowly through your engine compartment. Chances are you will find a small leak somewhere (you'll hear it rev-up a little).
Just a thought, been down that road and sometimes things like this are hard to diagnose given the fact that there are a lot of hidden areas such as intake manifold runners and AOS boots.
Good luck, and keep us posted of your findings.
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