Wont Start When It Rains

[tcracing425ex]

My dad has a 01 chevy 1500 6cyl. it will intermitingly not run when it rains. it happens maybe 2-3 times a year where it will rain at night, he will get in the truck to go to work the next morning and all itll do is turn over. we have had it to a couple shops who both thought it was the distributor, that has been replaced twice. it doesnt happen if he runs through a puddle or anything only when it rains at night. when it drys itself out it runs like a champ again.