If the car is new to you, could be a long list. Previous owners are known for compensating instead of fixing many things. Once you have all the vacuum leaks chased down, then I would start looking at vacuum hoses hooked up to the wrong place, and hoses that might be blocked. The O rings on my 83 fuel injectors leaked so bad that the idle speed needed to be turned down 100's of rpm after they were replaced, but the old ones didn't have any special symptoms otherwise.
"Jim Morehouse"
jim928@ptd.net has the WorkShop Manuals on a CD, and you will need them if you plan to work on the 928 yourself.
Sorry I can't suggest anything more specific. I tried a couple simple searches, and kept getting stuff related to problems on 32v cars. Running rich is often associated with surging, so you might want to think about electrical connections on sensors as well, the air flow meter and O2 sensors etc.