1.Clean the boost control solenoid with some good quality aerosol throttle body cleaner, sounds like it's sticking/gummy.
2. Check that the
bov isn't leaking (with the engine at idle, spray it down with same cleaner that you'll use for the solenoid, if the idles stumbles the
bov is leaking & knackered).
3. check all hose clamps between the turbo and the throttle body, tweak them all a little tighter.
Report back!
P.s Don't blow more than 9-10 psi down the boost control lines, you may rupture the wastgate actuator's diaphgram.