HI there, noted with initerest your post, and am having the same problem with my "N" reg A6. In 4th and 5th gear there seems to be a rev limiter effect that cuts the power at about 2,800 revs in warm weather and 3,000 revs in damp/cold weather.
The other day coming home it lost all of its power and then shot off like you described, but the morning after it would not start and has been at the garage since and they are now lost for ideas having checked the obvious usual culprits.
Did you get sorted and was it something simple that you can share?? I hope so as i am at my wits end now trying to sort it out!
Steve