Thought I would share this incase someone comes along with same problem. Went to two Audi dealers repair shops and they couldn't figure it out. A8 would stall (out of gas symptom) around 3/4 of a tank of fuel. I new this to be the case after the new fuel pump was installed. What was happening was the fuel switching (? not sure of the correctname) inside the tanks malfunctioned not the fuel pump. I don't know the best process for repair, but my auto shop had me change the tank since the fuel switching system is inside the tank (at least two of them are). Big $$$$

 

That sucks bro, if they replaced the fuel pump the main fuel sender is right next to it which mainly affects the fuel gauge which is most likely the problem. Its a shame audi techs accross the US can't diagnose/fix the problems on our cars that are so simple.