Check your shift linkage where it connects to the tranny. It's probably slipping and has a little slop on the rod that goes into the tranny. My shifter moves slightly like the way you described, and there is a little play in my shift linkage.
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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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