2006 Jetta TDI Black SMOKE

I have a 2006 Jetta TDI. I now have 240,000 miles on the vehicle. When the car was newer I got between 47-52 MPG. Over the last year this has dropped to the low 40's. I thought this may be due to the miles on the vehicle.
The other day I left on a 700 mile trip. About 650 miles into it my car suddenly seemed to loose power and produce black smoke under acceleration. The turbo runs more than normal. Once you get the black smoke "blown out" the car will run ok again for a while. I seem to have more problems with the carbon build up driving around town. On the interstate it seems to run better when I can get going 75-85mph.
I replaced the air filter. I replace the fuel filter. No better.
I read some forums written with similar problems. Sounded like the MAF sensor. I unhooked it. Seemed like it was running a little better with it unhooked. So I put on a new MAF sensor. Still really no better.
As long as I periodlically shift into neutral and rev the engine up to 4000-5000 rpm the car is driveable. But it still isn't 100% "right."
The check engine light has never come on.
The emissions light is not on either.
Kind of sounds like a vacuum leak? Is there any quick checks here?

If anyone has an idea of something to try next, I would appreciate it.