I'd suggest tracking down some of the annual supplements that Porsche produced for each model year. There's some good information in those.
Your problem is a little different. Sooner or later, when trying to sort out a problem like this, I suggest starting with the basics. Too often, we tend to look at specific components that we think could cause a particular problem. In the end, I think it would be quicker to just start with checking for vacuum leaks, fuel pressures and so on. Don't forget that the ignition system needs to be considered as well. Make sure that the advance/retard is free and functioning correctly.