My old car did the same thing. Its the most pronounced when the engine is cold and for the most part goes away once everything is warm. Started out just like what you described, and my problem got worse with time. Seemed to be the worst starting off from a dead stop w/ a cold engine. My car sounded like it was bogging or something, really weird, NO power while making that sound. One time I was at the bottom of a hill at a red light right after I cranked the car. I almost had to pull to the side, it barely had enough power to get to the top (This was at the problem's peak)
I Took it to the dealership numerous times. Hell, the Regional service manager came and got the car and took it to the place where they teach to be Audi mechanincs. Replaced items (lets see if I can remember them all): All 4 spark plugs, Mass air, entire throttle body unit, ECU, air filter, fuel filter, injectors, various cleaning/replacement of different sensors, etc..... End result. Yep, still did it. While not as pronounced/often as before the transplant
, it was still a problem. I got rid of that car, much to my service department's relief.
I hope I didn't just describe your cars behavior, for your own sake. Best of luck!!