Sounds like a vacuum leak to me. The faster your engine runs, the less effect a leak has on it due to the amount of air going through the engine.

Your vac leak could be on the inside - switching my climate control from off to econ pegs the needle all the way to the left instead of being 1/8" over.

Might check your vacuum reservoir as well. An idle that's real bad is usually a sizeable leak - try unplugging the vac line from brake the booster and plugging it (the line) with your finger and see if it smooths out any. Make sure to check for cracking on the hoses around your idle control and to check the idle control itself.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.