I had that issue when running fully stock. it was ultimately solved with a tune where the throttle response was improved. How many miles on your spark plugs?
2008 Metallic Alloy GT Convertible Premium
Show : black top| 18" bullitt polished wheels| CDC light bar and black billet replacement grilles | black/amber corner lights | GT500 rear spoiler| AH chin spoiler Go: FRPP CAI with StangHi tunes | Kooks LT's | FR500 axle back | Steeda Sport springs | Koni STR.t struts/shocks | J&M camber plates | UMI adj. panhard bar | LCA relocation brackets | GT500 LCA's | Hurst Billet shifter | 3.73 gear | FRPP axle girdle | Baer Rotors and Pads