has anyone ever come across this?

just put on a hybrid turbo, actuator, new dual chamber stainless elbow, downpipe.. the car is boosting at 1.4 bar until you go flap out and at about 5k revs it creeps til about 2 bar and power cuts.. its definitely not an electrical misfire. all vacuums and IC pipes seem ok.. im wonderintg has anyone had issues where the bigger compressor housing and the elbow not big enough to blow put all the gasses and the back pressure causes the spiking? if I could get an actuator handy id try that but there not easy got Id say for this turbo.. standard ECU , injectors, car has 1.2mm cosworth head gasket and adjustable fuel pressure reg, full exhaust and induction kit is all the other engine mods..

thanks