Could be your synchros on their way out, or you are shifting too fast for them. It could be a clutch, although mine did it with stock and my new ACT as well. It doesnt do it all the time, just sometimes when I shift on the faster side.
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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
Team B.O.B.® - Ballaz on a Budget