alarm on 1998 golf sounds 10 mins after locking?

Hi Mike. My wife's Golf III had the same symptoms. The PCB for setting the sensitivity is located in the roof behind the courtesy light. Try adjusting the little trim pot until you hit a sweet spot. Mark the orientation before you start adjusting. Arm the alarm with yourself in the vehicle and then adjust it one way until you see it becoming more sensitive. Then adjust it back to your reference postition and a little bit further. It might take some iterative fidgeting with it. I have a theory that the components on this PCB is subjected to higher temperatures than the rest of the car and so "age" more rapidly.

Anton
December 2008