Hey guys wat up?

I've been having this problem for a long time. After it rains/snows extensively my caprice starts acting up. It starts fine, and makes it through a mile or two without any problems. Then all of a sudden as i slow for a stoplight/whatever the engine revs up and down, and continues doing so until it shuts off. I've had the distributor cap replace, also replaced the spark plug cables. Today when my car started acting up i shifted into park-and for some reason the engine corrected itself and ran normally-does that give you guys any clues? I'm really stuck here, i appreciate any help.