Is this a feeling that when the rev counter goes over 5k it feels like the turbo is running out of steam and then as you approached close to 6k it power comes on full again? I have studied my vRS with a KO4 fitted and i have noticed that it does this ever so slightly in 4th gear only going up an incline, not enough to bother me to be honest. It's obvious the ECU is seeing something it doesn't like and pulling back, maybe a sensor seeing high temps?