Brian,
I didn't get to read the responses, you got to this problem, but I thought
I'd throw out my theories and guesses.
My favorite guess right now is condensation in the fuel system. You might
want to try adding some alcohol-based water remover to the tank.
Unfortunately, the other part of my favorite guess is that the condensation
is occurring in the fuel bowl, and I wouldn't know what to do about that.
You could open it up and look, but it's probably just a waste of time.
Well, this seems unlikely now that I've written it, but you seem to be
looking for all possible explanations.
Gotta go,
Lee