2001 Volkswagen Jetta Car runs poor when wet outside?

I have had problems when it rains or is very humid out. The car starts and runs very poor for the first 3or 4 miles then clears out. The check engine light would come on and flash the it would go off. Knoe the light has been on for a while. To it to the dealer and they said they cuold not find the problem. I've had other issues with this dealer in Virginia Beach before. I know there are some Catalytic Converter mound. I don't know if this could be related. I am taking it back to another dealer next weak. I just would like some thoughts so I do not go there clueless.

Thank you.

Scott