Evenin' all. I've just been to see a 1991 R129 SL500 I'm interested in, and it has (among others) an idling issue. The engine idled normally at first, but then rose to idle at 1500, then 1700, rpm. If I manually blipped the linkage at the rear above the engine, then held it against the stop, it would drop back towards normal idle for a second or two, then up again to 1500-1700 rpm. Does this sound familiar to anyone? Any ideas what it could be?