1993 Pontiac Bonneville engine cutting out

When weather is wet, or moisture is in the air, my wife's bonneville starts cutting out, or missing out. I have changed sparkplugs recently and that helped for about 2 weeks or the next raiy day came, and then problem came back. Could you give some ideas what it is? Also my wife has been looking on the internet too, and found some tips for me to try, like change air filter( I can dothat), change gas filter( I can't seem to find that). So could tell where to look for that as well? Thank you! Richard Vianco