Try spraying some water on the back of the engine when it's running (like from a squirt bottle) at the point where the stator wires come out of the engine case. If that doesn't recreate the problem spritz the spark plug wire all the way to the plug. This sounds like a typical wire short that is occuring when water is introduced to worn, split or cracked wires.
Had a similar problem on a Kawasaki 2 stroke that was due to a pinhole in one of my spark plug wires that would arc to the gas tank when it got wet.
I find it a bit hard to believe that a wire would have cracked in only 3 years. It seems much more likely that either a wire has worn through or a fitting has gotten corroded.....
Lastly, before buying a new stator you might pull the fly wheel and the stator and look for bad wires between the stator and the connector on the back of the engine. I recently had to replace several on my p200e- simply soldered new ones on, not hard. Hope this helps.
1980 Vespa P200E