vauxhall zafira, 2.0 dti?

hi, just bought a zafira 2.0 dti, it seems down on power and when you release the throttle pedal the engine continues to rev high and fall slowly. it dosent seem to happen when you rev the engine when the vehicle is staionary. any help grately appreciated, i know my way around a car but fault diagnose is not me strong point. thanks....craig