2.5 a6 tdi sport power loss/ smoking help!

[Richrad]

I currently own a 2003 2.5 tdi A6 sport. this problem has happened 7 times in the last year when cold, it starts fine but immediatly heavily labours chucks thick black smoke out the exhausts and has no power it will not move at all! after a period of about an hour turning it on and off it then becomes driveable and all problems seem to clear apart from an occasional loss of power when driving but this seems to then clear itself and the car will lurch forward again and return to normal power. any ideas please i am completely stumped so are audi and the local garage!