Turbo actuator can cause the issues you discribed. Often intermittent however sounds like yours has thrown the towl in.
So your driving with the waste gate open all the time and not building boost pressure.
Sounds daft but could be cured with a throttle body alignment it can drop out of correlation and it relays on throttle position to give an accurate amount of boost to correspond to with throttle position/requested power.
Failing that I'd hazard a guess at a new turbo actuator motor
