I've had this issue on my GT. After trying new ignition system i had it dyno'd and it was the actualy AFR that was the issue. Lack of fuel under boost was causing it to hesitate and stutter/misfire really badly. knocked the fueling up slightly and fitted a better rrfpr (with gauge to assist adjustment). then tried it on the road and did that until it ran smoother, but had it mapped to cure it (and we all know how that ended... lol...)