Basically im in the same boat i have a 1997 a4 b5 v6 2.6L audi
i dont know what is wrong with it nor do mechanics basically heres what happened:

it was stalling after i started the car it would idle and just stall (its an auto)
then it got a bit more frequent and started over reving when it was trying to change gears
until one day i was driving and i heard a clunk and it wouldnt accelerate and so i pulled over let it sit then it wouldnt start it just kept turning over and ideas what it could be and how to fix it?
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Timing belt. Needs a new motor probably