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.