The A2/Mk2 VWs have a big problem in wet conditions for some reason. Are you hearing any squeeling coming from the belts? Are you purging the cooling system when you are adding water? My old 92 had a lot of problems with the fan switch located on the radiator on the lower left side. It could be getting wet in the rain and causing the engine to boil over. Also, don't just keep putting water in the cooling system this is a sure-fire way to pop your head gasket. Make sure you are using at least a 50/50 water/antifreeze mixture.