Check all your vacuum hoses and connections for leaks...vacuum leaks can cause an erratic idle. Does it still happen while the car is warm? The fast idle thermo valve only controls idle while the engine is cold. Also could be a faulty/stuck IAC valve.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
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