NineMeister

Because the problem only occurs in the higher gears your system is on the very edge of its control envelope, so a small improvement will probably do the trick as long as the overboost is not more than 1psi. My recommendation is to first strip the wastegate and make sure that it is in perfect condition (especially the diaphragm) and if all is ok look into "porting" the inside of the wastegate housing in order to allow more gas though at full lift.