With use, the bore gets a buildup of carbon around the butterfly valve which messes up the idle. To clean, remove the throttle body and clean the bore and butterfly valve using carb cleaner (or throttle body cleaning spray) and a toothbrush. You'll have to work at it...the stuff is hard to get off. When clean, you should see an even gap of air around the butterfly valve.
95 E320 Cabriolet, 131K