audi 80 tdi chugging

Hi i'm new here can anybody help

I have a 1994 audi 80 tdi with 111,000 miles on the clock
The engine is lumpy at tick over and when driving at about 1700 to 2200 rpm the engine starts chugging
If you accelerate harder it goes away there is no lack of performance
There are no fault codes showing on vagcom

Any comments would be greatly appreciated

cheers beaver