98 Saturn SL1 - runs poorly during wet weather

I have a 98 Saturn SL1 which has an odd problem. When the weather is nice (ie: hot/warm, no rain), then it runs fine. However, as soon as it gets colder, and especially when it's cold & rainy, it runs very poorly. Acceleration is difficult & jerky, and one can clearly hear that the engine is running rougher. At these times, the "Service Engine soon" light comes on, sometimes intermittently, sometimes for the whole trip. However, if the weather clears up and becomes warm again, all is well. If anyone has the same problem (and has discovered the reason why), please advise.