[gregwils]

Ok, this isn't my area of expertise, but after reading enough posts I can probably suggest a few things. My first guess is a vacuum leak. A hose that cracked or split could cause a quick change in performance. If you can spray some carburetor or brake cleaner around the intake manifold and around the back of the engine, and if the idle picks up speed, it's a vacuum leak.

You may also want to check the cylinder head temperature sensor. You can test to see if it's good, check Bentley. Also, make sure it's the newer two wire type. I don't think the car would run if the speed or reference sensors are bad, however, if they are old (protective cover is brittle, temp sensor too) then I would add them to your to-do list. You can get BMW parts for speed/reference from our host at half the price of Porsche part numbers. Do a search.