328 Runs Bad When Its Raining Out

* ACCIDENTALLY PUT 328 FOR TITLE, I MEANT 318*
My truck runs fine on a dry day, but if it's snowing or raining it will completely change. It won't hold the idle, I have to do two-foot driving to keep it from stalling every time I decelerate to 30km/h or less because for some reason the revs will drop way too early, it also doesn't idle on its own in any gear, and sometimes when I'm driving, the revs will actually suddenly jump too high, accelerating the truck without me having my foot on the gas at all (which is dangerous) Then I have to kick the gas pedal until the revs drop back down again. I have a new throttle cable, so it's not that.
Any ideas on what might be getting wet when it shouldn't be? There has to be some sort of leak somewhere but I don't know how to look for one. And some people might think this seems like a weird or non realistic problem, but I kept track of when it ran rough, and every single rainy day in the past 30 days, it ran bad, And every dry day, it ran fine.

Thanks in advance

when was the last time it had a tune up? distributor cap/rotor/wires possibly ballast resister.

Agree, crack in distributor cap or bad/cracked plug wires.

X3...crack in distributor cap.

Thanks everyone I will check the distributor and dist cap and wires. Will let you know how it goes

At night, grab a mist spray bottle, dark of night, pop hood. Spray water MIST into the air above the engine. Look for blue arcs in plug wires and or around cap...this may help.