2001 Chevy S-10 misfire in the rain

My truck has had a misfire for about 2 years but it would only misfire when the truck would sit in the rain not running and the engine temp was under 100 deg. After the truck sat in the rain for a few hours I would start it and it would run real rough for a few miles and then it would quite and run normal. During the misfire somtimes the check engine light would flash, light stays on all the time. Never had addressed the problem because it only does this when it sits in the rain. I can be driving the truck and hit a real good rain storm and it runs just fine never misses. Took the truck to the car wash last week and hosed it off, and now the truck misfires non stop. I am sure it needs plugs but this has been going on well before 100,000 miles. Misfire has always gone away before, but now it is here to stay. Any ideas would be great.