Actually I just re-read your post and noticed you said that it runs fine when warmed up, and you don't have the problem when it's warm...so it's not misfiring, no fuel/spark/timing belt problems. It could be the fast idle thermo valve, which controls idle when the engine is cold.
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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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