Truck will not start with too much moisture in the air?

Problem is getting worse. Seems like it wouldn't start very well (idled rough for a few seconds)if there was a lot of moisture or if it rained over night. Now it won't start at all in the morning if we have any moisture/rain. Starts in afternoon fine. Seems to be only after long sits (overnight). I have less than 2 years on fuel pump/filter, plugs and wires, distributor cap and rotor. I have tried taking cap off and drying out, and even put plastic over cap incase it was getting wet from rain. Problem is getting worse.