Moisture in the kickstand switch will make it sputter as the switch wont work like its all the way closed or open. When you drift it to a stop while driving in gear you are also flooding it because you have manually fed carbs but no fire.
I strongly believe its 1- electrical and 2- low on the bike and forward. (thats because thats where most the water is hitting.) unless the headlamp itself is leaking which I doubt.
anyhow, good luck on the trip and be safe. Take it in when you get home and all will be well.
Have a good vacation!
