1998 Chevy Express starting problems

My 1998 Chevy Express with a 350 engine and 4spd auto is having starting problems when it rains. The truck runs fine once it starts except for a crazed speedometer. The engine smoothes out after a few minutes but the speedo takes almost a day to get back to normal. This only hapens whenthe truck sits out in a rain or if it is foggy. New plugs, wires, cap made no difference. Any Ideas?