O.k. so why is it still not starting?

o.k. i have a 1991 chevy k1500 4wd 4.3l v6 everytime it raines or is wet outside it does not start. i replaced the distributor cap rotor wires and is still doing the same thing... coldest week of the year around 7 degrees it started every a.m. with no problem it got warmer things started to thaw a little and nothing. please help.