About 18 months ago when i first bought my mk3, it started randomly cutting out on the move. I took it to the dealer who said I had to have a new immobiliser fitted. My dad said this was crap, so I went down to the local specialist who said it was the ignition barrel. I got the barrel changed and have had no cutting out since!
Mine is a '97 gti 8v with 67K on the clock.