I think you are on the right track by draining the tank and getting *all* the water out of it. Of course, that may not be it, but..
(settling in to camp chair around the fire)
Let me tell you a story. One time a guy asked me what was wrong with his Luscombe. His little 65 horsepower Continental would purr along.. hiccup.. then continue purring. He said he was getting afraid to fly it. I said, "You didn't drain the carb at the annual inspection like you are supposed to, did you?" "Uhh, no." We did that, and he lived happily ever after.
Water is funny stuff.