1999 Chevy Astro Van - Running rough and stalls out in rainy/snowy/wet weather!?

1st happened in the winter a few years ago... it was snowing and it started running really rough before it stalled out on the highway. I sat for about 10 minutes and it started back up. Drove for about 2 or 3 miles and it starting running rough again. Repair shop said it had some bad wires and plugs and they gave it a tune up. Still did the same thing in the wet weather. Had the distibutor cap changed...no luck. Eventually had the wires and plugs changed again, along with the cap again... still nothing. Somebody said something about a module or something under the distributor cap would probably fix it. Anybody have any clue what they were talking about? The guy acted like he knew what he was talking about... like he had the same problems and this fixed it. HELP PLEASE!!!