Following a later post
http://forums.pelicanparts.com/porsche-928-technical-forum/845731-high-idle-rich-running-occasional-misfire.html
I sprayed bursts of carb cleaner around the base of the throttle body, the rubber sleeves on the spider, etc. I noticed no change in the idle speed, so I'm hoping that I don't have a vacuum leak anywhere.
I went ahead and adjusted the large idle screw clockwise to bring the rpms down until they were jumping between 820-830. I then pulled the air cleaner off to get at the 3mm screw. I turned it counter clockwise until the engine started to stumble, then clockwise until it started to stuble, and then set it half way between the two, favoring it rich just a hair. I found those instructions over on the 911 forum. I took the 928 up on the highway for a a 4-5 mile drive, and all seems okay. One unexpected benefit of not havong a high idle anymore is that my syncros don't grind like they used to when shifting gears.