Toyota starts run crusty and miss when something gets wet?

When I drive on a dry day it runs fine, but when its wet out my truck wants to die out and starts to miss fire. I put on new distributor cap, spark plugs and wires. My uncle told me to put a bag over the distributor to see if it would help but it still happens. I'm thinking its under the engine because when I pour water on the top of the engine it still runs fine.