It sounds like your stuttering under full boost coz im having the same problem with my S2, ive been told the gaps on the spark plugs are to big and the sparks being blown out under full boost, or its overboosting coz its not chipped and the actuator needs "turning down" back to 7-8psi. Mines running 10-12psi and stutters but if you feed the pedal slowly it goes with out stuttering. Im changing my plugs, leads, fuel fitter etc to find if it is the gap on the plugs, or leads or fuel filter, if not its gotta get set up by a tuner !! Hope this helps

MattyRST