New to this list, so I hope this post is in the right area ...

I have a 2003 Audi A6 TDi about 50,000 miles on the clock and serviced less then 1000 miles ago (at dealers). When the throttle is constant (or with cruise on) the power delivery is not very constant, revs don't change but you can feel the car wavering in delivering the power. It's also giving quite a bit of black smoke too, not ALL the time, but I never really get any on my Passat (with a similar engine). Acceleration is great and smooth, it's just when on constant throttle.
Any ideas what the problem could be??