I also have the same issue with my 1996 A4 TDI.
I've just posted in this thread some details of what I have tried, which may offer some help.
Someone must know what is causing this. Please!
Mine has 126,000 miles on it, and I have just had the manifold cleaned out. It was blocked, but still the problem is there!
Bit embarrasing when Austin Metro's go flying past you...
Is anyone else seeing oil in connector which goes from the ECU (top right of the engine) to the Manifold. Mine was blocked with oil and the ECU contained quite a bit of oil. A suggestion is that the diaphagm in the ECU which deals with air flow could be clogged up with oil. Is anyone else able to check?
Other suggestions have been a poorly diesel pump, but I can't afford to go there. Mrs Miggins in her Austin Metro can overtake me as much as she likes for the price of one of those...
This TDI power loss thing is becoming a bit of a club, but not one I'm that keen on being a member of!