fuel pressure problem
I have a Porsche 928 with CIS, and had the same symptoms for a long time. I didnt do anything until it got worse and lost all power and response. I got a CIS gauge set from JCWhitney (~$60), and a book from Amazon - How to tune and Modify Bosch EFI by Ben Watson - ~$15. This covers CIS, and will tell how to test everything, gives all pressure settings for CIS on all models it was used on. My problem was a blocking Warm up Regulator, producing high control pressure, giving lean mixture. Watson doesnt tell you how to fix WUR or fuel distributor, but the WUR is not hard to work on at all (easy to say now, I was a bit wary before though). As soon as I got pressure right, idle came up beautiful, response is superb.
Get the book and gauge set, get the system and control pressures documented, then you know where to go. If your control is more than say 60% of system pressure, you will be running lean is my guess.
Email me if you need more.