Hi mate i am having the same problem with mine so yesterday went to the gurus at graham Goode was there for four and a bit hours i have to say these guys now there stuff, Alistar took the car out with lap top attached and after a few gremlins where sorted out the car was still misfireing at the high end of the boost scale, Turns out my fuel pump cant cope at the high end and the ecu was trying too put 50% more fuel in which is way too much this in effect was also causing my car to stall out when coming down to idle as the ecu was still overfueling too compansate for the high boost,so new fuel pump on order and the car will be booked back in at Graham Goode.
Also have too say i was there as mentioned for over four hours and because i took the car away effectifully not fixed although the problem was identified the guys or more Alistar only charged me for 1 hours labour and 1 hours diagnostics time how good was that. this level of service and respect for the customer and his car does,nt happen enough in the car tuning ,service game big thumbs up for Graham Goode.