Don't know if this will help. I was dicking with the idle set screw on my ICV about a year ago. I closed it up a bit (clockwise) to get another 100 rpm. A couple of days later I went for a cruise on the highway, no problems at speed but when I pulled off the road to stop the idle was swinging from 700 to about 1900 rpm in oscillation not unlike the one you're describing. This was the only change I'd done. Backed the set screw off (CCW) and everything went back to normal. I read later it has something to do with a bad mixture that the motronic corrects - the icv reacts, the motronic corrects, etc. Thus the oscillation. Don't know if this is right and there may well be a gang of opposing opinions, but I include my experience in hope of providing a way of fixing an irritating problem. Best of luck.

85 528e