I recently bought a BEX 190hp A4 quattro as above, and after having it revo'd have got surging and generally bad boost after 200 miles.
The technician at Revo has been really helpful and has pointed the finger at my n249 valve. The issue is i cannot find it and my dealer says it does not have one fitted!!!!
Under the inlet manifold the car has 2 of 026906283H fitted to it which are linked - which i got as EGR solenoids, but my dealer says are 026 906 283J now (which seem to be a N75 - which i HAVE found i think) lower in the car.