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Hi it sounds like either a turbo or mass air flow meter or a vacuum leak.
A good test is to open the bonnet and look on the inlet manifold and you'll see a vacuum operated flap, start the engine and allow it to idle for a while. Then switch it off and watch this flap as you do it, it should move and you'll also hear it and then after a short period it will return to where it started. If the=is fails to happen you have a vacuum leak somewhere and you'll need to check all the vacuum pipes. If it moves then the vacuum system is ok and you have a fault with the turbo of mass air flow meter. You can check the actuator operation of the turbo by watching it when you start the car up and as you rev it as this arm should move. If it doesn.t move then you could well be looking at the turbo.
There are now several companies offering rebuilt turbos for these so it would pay to shop around before you buy one.
I hope this has helped you.