stuttering and black smoke

[riebs]

I have a 2000 1.9 TDI auto,  and have had some problems recently with stuttering and black smoke after I start the car.

The car starts fine and idles fine for about a minute or two, but shortly thereafter it begins to stutter and rev intermittently (to about 1500 rpms).  

Once I start driving and give it some gas, it continues to stutter and spew black smoke, but only for a few minutes (until the turbo kicks in? I think?) and then it begins to act normally, but only until I turn the car off.  

Once I start the car again, even while the engine's still hot, it will start acting up again, but only till I begin driving.

I have a CEL on, but haven't been able to get it diagnosed yet.  I'll try and get that done tomorrow, but I've also noticed a significant decrease in the gas mileage.

Any clue as to what it might be that's causing all this?

Thanks