You either have a vacuum leak or a faulty sensor. The easiest way to find out is to plug the car into a Porsche scan tool and look for CEL codes or run the sensor tests.
It sounds like a temperature sensor or a throttle position sensor to me.
__________________ Cogito Ergo Zoom!
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Current Porsches:
2004 Cayenne Turbo
2003 Boxster S Past Porsches:
1989 911 turbo
1981 911SC
1984 944
1973 914