Misfire?

Every time it rains, I will get intermittent bucking mostly when I am first starting to move in 1st gear. Also, at idle, the RPMs bounce between 500-750. My first thought was condensation in my gas tank (it sat for a a few months with a half tank of gas...seemed plausible) so I added a few bottles of Heet, but i noticed the bucking only happens when it rains. I did a tuned up about a year and a half ago. new plugs, wires, cap and rotor. Any thoughts? UD is expensive so I would like to start with the cheapest thing first.

hmmm it doesn't seem like anything electric is getting wet, my cap and rotor both seem dry for that matter. if I had to guess it would be a humidity issue