I've been pursuing similar kinds of weirdness in my '89 535 manual. Idle out of control, intermittent and unpredictable fuel cutoff above 1500 RPM. I've checked nearly everything in the FI system and have found no obvious smoking gun. There is no error code from the "stomp test". I even pulled out the ICV and cleaned it with some small improvement. So it looks like either the ICV has intermittent failures/hangups as it heats up, and or the ECU is giving it improper commands. I'm going to give the benefit of the doubt to the ECU since it comes up clean in the self-test and take the plunge with a new ICV. I should know within a week when I get the new ICV whether my diagnosis was correct. That might give you a datapoint on your problem.Originally Posted by George Davis
As a minimum, try taking your ICV out and cleaning it. It's a 10 minute job. I took Saran wrap and blocked off one nipple, poured in a little Gumout, and then capped off the other nipple with Saran wrap. Shook it around a while, including some rotation around its axis. It looked pristine clean afterward and seemed to rotate freely, but my problem was only somewhat improved.