Have a look at the last page or two of my Coupe GT thread. Mine started out with occasional stumbles when flooring it and got progressively worse until it cut out completely and wouldn't restart. I was convinced it was fuel at first but realised no spark and think I've now tracked it down to a faulty rotor arm losing spark through to the distributor shaft. Unfortunately I've not been able to get to the car since I ordered a new one to confirm it yet, but it's definitely worth a look... mine has burn marks just below the rev limiter so it looks as though that's the problem. The thread will give you other ideas of where to look also...
http://www.classic-audi.co.uk/forum/...=39834&page=11