2002 Mazda MPV Engine runs very rough in rain

We were told by the dealership that our check engine light was on because of probable need for a new transmission. They could not at the time be definitive about this as the codes the engine was throwing were not easily interpreted. We took the car to another dealership and also to a private transmission repair center all with the same recommendation that we replace the transmission without a guarantee that this was the actual problem. Needless to say the car has 146,000 miles and we are not going to spend the money for a "fix" like that. The car intermittently runs rough especially when it is raining. Any idea if this could be related to any other easy fix we could do (and not be the transmission)?