Had the same issue a few years ago on my 3.0 QS Avant. No fault codes but issue got worse when car was warm. Dealer replaced ignition switch didn't fix problem. Needed new module under dash about £300 if I recall correctly
After 5 years time has come to get rid of my 3.0 QS avant.
Have sold it on to the garage who did all my servicing/repairs.
Have now bought a Bmw 325i M Sport Touring which has all the toys I wished I had with the Audi.
Thanks to all who helped in the past and I will continue to look in...