The problem you are having is the common GOLF MK3 Syndrome.
Basicly, as your slowing down or changing to a lower/higher gear. Your car drops revs and sometimes cuts out.
I had the same problem once. The fix for this is NOT the relay number 30 switch. It is to simply clean the throttle body with some carb cleaner. The best way to do this is to go to your nearest garage and just ask can you unclip and claen the throttle body. Its a 2 minute job.
Why this happens with the Golf MK3?
Because Golf Mk3 Haven`t got a cat flap filter like most cars.