vauxhall astra diesel lack of power from turbo?

anyone had this problem with a 2.0 astra g diesel.1999.
whats happening is im getting the engine managment light come on when driving,at first when i put my foot down the light goes out but if i give it some stick (high revs) the light stays on & i lose all turbo power.when the cars been switch off a while everything goes back to normal.well at least untill i drive it again.ive cleaned the egr valve out & thats working fine.all vacume pipes are good too.we have plugged it in a scanner & got these faults.egr system fault & turbo boost valve fault sensor A.anyone got any ideas?