93 Ramcharger help

I have a 93 Ramcharger 5.2 magnum A518 trans. Well it runs good for the most part, but in rainy or misty weather it will have a noticble performance decline and it will backfire all over the place if the rpm's get too high. I thought it was a bad cap, so it did cap/rotor/plugs wires. Ran great until the next time it rained and started to do the loss of power and backfiring again. Any suggestions on where to look to fix this. I bought the truck for my winter vehicle, and it needs to be fixed.
Thanks!