To Made in Germany- I don't know the solution so I don't know if it relates to your engine. I also don't know about the cost of the idle control valve. I am guessing very expensive as someone is trying to borrow one to test their car at another string in this forum.
Did your idle go high over a short period of time? Someone suggested a lot of little vacuum leaks around the injectors which would be common over different engines, but I would think that would take quite a while to develop.
I have been busy doing brakes, shocks, and trans fluid and filter but plan to get a scope and check waveforms among the three computers. I suspect something is giving it too much air and fuel. I find it interesting that three of us have the same problem in the same vintage cars. Our cars have had plenty of time for connectors to oxidize, that would be a cheap fix if we could find the right connector. Please let me know what happens with your car.