Check your install. Sounds like you dont have it mounted and sealed well. My kit car used to backfire between gears when the doughnuts between the manifold and Y-pipe weren't sealed well.
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07 Solstice, black, 5spd manual, convenience, preferred, and premium packages, LSD, Monsoon, XM, A/C, ABS, chrome wheels.
Mods: DDM Stage II supercharger. Solo Performance shorty header, hi-flow cat, and SQR-2 catback. Norms rear fascia. MSD DashHawk.