went through the same thing. here's what ultimately
solved my problem: http://benz-store.com/idle_speed_control_unit.html

it's rebuilt and adjustable. you'll also want to check your
air slide valve to make sure it's working and check resistance.
should be 8-12 ohms. if it sat, the old junk may be gumming up the slide. clean it out with carb cleaner.

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rayroyw
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'85 380SE (238,300) my "new" old car!
'84 300D (303,000+)
'72 250/8 (mercy heaven)