[Wayne 962]

One thing I always like to do is to check for vacuum leaks, which can cause annoying problems like these. I've got an article on using a smoke machine (very useful tool) here for the 996. That would at least rule the leaks out before you start replacing expensive parts:

Pelican Porsche Tech Article - Finding Vacuum Leaks on the Porsche 911 Carrera - Porsche 996 Carrera (1998-2005), Porsche 997 Carrera (2005-2008)

-Wayne