Smoking A4 Estate

[pastemaster34]

Bought my first audi a few months back and its now losing power in 2nd/3rd gear and sooty black smoke is coming from the exhaust.Once car has picked up power the smoke has gone,but then comes back again when under 2000 rpm....

Car is 2005 2.0tdi 140 bhp version Avant...

Any help please

kevin