check the pipe off the inlet manifold to the brake servo as if this becomes loose in the fitment to the manifold it starts to do this and eventually comes out and you may as well have anti lag lol

mine did this and it was holding the revs up at 5k with the turbo chattering and etc lol was funny but not good as everything i did and wondered if i had fixed it ment i had to start the car from cold to it doing this :O