Definitely the cat...I had the same problem, and the same problem with an OBD2 cat. Magnaflow/Carsound makes an OBD2 legal cat.
http://www.car-sound.com/catalog/universal/430.aspx
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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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