A6 TDi sport loss of power/ smoking help!

I 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 at all it will not rev or move at all! after a period of about an hour turning it on and off it eventually becomes driveable again and all problems seem to clear apart from an occasional loss of power when driving but this seems to then clear itslef the car will lurch and then return to normal power. Any ideas please i am completely stumped so are audi computors and the local garage!