'88 S-10 2.5 TBI. On a nice dry day will purr like a kitten, the minute it rains, surging and sputtering and then death. Full tune up, new computer, new MAP sensor, EGR working fine.Anyone else with this problem??

could be a distributor cap

I would agree and start with the distributor cap. Pull it off and ensure that it is dry and that neither it or the rotor are worn. If they are good you can either wait till it rains next to pull the cap off and check to see if moisture is in it, or try and figure out where the rain is coming in from. You might have missing weatherstrip on the back side of the hood that isn't sealing right. If the water isn't coming in at one spot I've seen guys make a secondary cover for the cap or to seal up the cap with silicone.