sounds like your over boosting ...like i said at first ,do it careefully and gradually....follow this guide here but change to your required boost....

1-remove actuator pipe and set actuator at 1mm preload,then run at full boost and record result
2-wind in actuator rod 1/2 a turn at a time and run under load until the boost held is 26 psi...you now know that your actuator will hold 26 psi and not be forced open by exhaust pressure
3-replace actuator to amal pipe and when i run again boost held had dropped to 17 psi
4-i then put 1mm restrictor in amal outlet to actuator and run again,boost was still 26 psi held which meant not enough pressure was going through restrictor to open actuator.
5-drilled restrictor to 1.5mm and run.... boost went to 26 psi then dropped to 24psi held
6-drilled restrictor to 2mm and run....boost went to 26psi then dropped to 23psi held....job done