I have exactly the same problem, and that's on a 2005 model with a NEW fuel injection pump, suipplied and fitted by Auidi. We've a good garage local to us & he has all the kit to read the fuel pump codes. Ours has an E54 (I think) and an ?86. The E54 is related to a small internal valve that is linked to the idle speed but as it's stuck it will affect power through out the power range. The ?86 is related to an internal Electrical fault.
The garage has cleared the code and we're going to monitor this closely.
My limited experiance on this subject would advise you to get a proper diagnostics done, should not be expensive (except and an Auidi dealership!) but would rule out "Turbo's" and the like....
Good luck! Would be interested to see how you get on!