I hope this is the right place to post.

My car has for some time exhibited the following symptoms which is driving me up the wall. I have been reading many threads here and on other VAG forums, but I did not find any good match.

Periodical lack of boost, engine noisy (throaty like under WOT) and slight shuddering (you can just feel it in the steering wheel, but the windshield visor vibrating is a giveaway). Most noticeable from 1400 – 3000 rpms. The symptoms are totally random in respect of when it appears, for how long, speed, gears and also variable in strength. I estimate loss of power to be varying between 0 and 20%. Onset can be instant or slowly increasing and decreasing. (Feels like a monkey in the engine bay playing with a power potentiometer). I can drive into a roundabout with lack of power and suddenly it flies out of the roundabout like a bat out of hell. When it has no symptoms it drives very well, very quiet (you can hear the hum of the fan over the engine) , loads of pull from 1000 rpm and up despite having 188 000 miles on the clock.
Actually I think there could be (at least two issues) as I can have both low boost with shuddering and also high boost with shuddering.

So far changed MAF (only improved mileage), air filter, DV and PCV. Coil packs was changed last year on recall, spark plugs are new and when pulled they look good. Use injection cleaning additive regularly. Oil consumption (long life) is not bad but fuel consumption (98 octane)around 25 mpg (mixed) seems low. At least once a week I have a 400 mile highway trip where it runs above 3000 rpm for long periods.

The car has been checked many times, never any codes, nothing in registers. Nothing, not even when running bad. My local (knowledgeable) mechanic and several Audi dealership mechanics are scratching their heads and offer no solutions.

Since there are no error codes I am thinking mechanical issue. I read a tread about loose waste gate which could be verified by a loose rod between actuator and waste gate arm, but my arm feels rock solid.

Comments and ideas are highly appreciated, thanks.