Randomly replacing parts isn't a valid method for troubleshooting an MFI system!
The best thing you can do for your car is to plan on doing your own work ... begin by downloading, printing, and reading all of the MFI documents on the following Pelican page:
http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm
As you familiarize yourself with the MFI system, build a special service info binder & library, and gather some specialized tools and test equipment, including a Gunson Gastester. Do a search of the Pelican archives ... there have been many threads on practically every aspect of the MFI system, including troubleshooting, tuning, and repairing. It wouldn't hurt to gather threads and info on the CDI system as well!
The central, and single most important document is a factory MFI publication called
Check, Measure, Adjust. There is a specific, ordered list of tasks to be followed in order to rectify MFI system performance.