I have a 1.8T, 98. At about 65,000 I started putting in non-premium gas. After about a year I started having quite a few issues with the vehicle rough idleing at stops, at times it would even stall at traffic lights. Restarting the car at these times could be difficult.
The Audi garage told me that my spark plugs were really poor and replaced. The car would drive somewhat better for a short period of time but then start the process all over again.
I then started using premium gas and changed the spark plugs again. Three years later I now have 129K miles and it runs smooth, no more cutting out or idle issues. I am not saying this is your problem but for me it was not using premium fuel.