I have had this car for more than five years and there is one annoying and persistent problem. At low engine speed <2000rpm and very light throttle pressure the car feels like it is running too lean and is fluffy and juddery like a misfire, give it more throttle and it pulls away perfectly but around town at low speed it is totally unacceptable for a car like this, even passengers can feel the misfire/judder. Everybody who tries the car agrees it is not right including Audi but when they could not fix the problem they sent me away saying the car is in spec and if I did not like it buy another car!! In the past five years almost everything has been changed except the car! It has a new ECU, GCU, cruise control CU, injectors, HT coils and leads, all the switches/detectors that anybody can think of like mass air flow, oxygen and the like have all been replaced but it still misfires! We have considered other causes like wheels and tires and drive shafts and all have been changed or checked.
I have convinced myself that the car is actually in spec and nothing has failed/broken but during the design/development the ECU fuel profile was set as lean as can be for fuel economy at this low engine speed/throttle setting and on my particular car it is just too lean to support a clean burn.
Hoping I could fix it I had the ECU "chip tuned". The outfit that did it said they dont change the fuel profile but I thought maybe it would help. The chip tune made a very marginal improvement to the pick up but it did nothing to improve this misfire.
My questions are:-
Has anyone had experience of this problem.
And has anybody got experience of making more radical changes to the ECU than a simple chip tune, maybe someone out there has the Audi software for the ECU and maybe had experience with it during development or in the aftermarket for more serious tuning.

It was a cracked vacuum hose under the engine cover. Check them by squeezing them. Sometimes the crack is small, not visible, but the suction will open it up.