Could be the solenoid that is faulty, or maybe there's another small vacuum leak elsewhere in the vac hoses. It might be worth removing and cleaning the boost pressure sensor (or manifold absolute pressure sensor - some manufacturers call it by a different name) too.

To clean it, use either electrical contact cleaner or carb / choke cleaner, both usually available from high-street discount stores for 99p or £1, and just hose the crud off the sensor, try and keep the chip / connector away from the spray.