#1 go to, have a good mechanic check the connection to the wastegate from the boost regulating solenoid. It is a rubber hose and can rot, stretch, get oil in it and not actuate correctly.
Next in line the electronic blow of valves are also easy to wear out and are sensitive to grime, if they stick open boost will not build properly..
loose hoses on the intake can also be a issue, check the service history of the car, if the car has had a lot of work something may have been cracked and or loosend..