1998 Chevy Express hard starting

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?

I have the same problems and a local mechanic told me my O2 sensors were bad. But he couldn't locate a few readings so I never let him finish the job. My van will suddenly bolt and shift into a lower gear when driving, this is followed by the speedo dropping to 35, even tho I know I am going near 65 mph. Only happens when it rains and first noticed in Florida during a downpour. I pulled over at a service station, the tranny was running real hot without scorching and the fuild read low. I added more and the problem seemed to fix itself, but upon checking, like I said,
the O2 sensors were bad. Talked to several mechanis who couldn't agree whether there are 2 or 4 of them. Good Luck!

The computer boxes corrod open and they get wet. They a bitch to get out but I pulled mine out and it was. It had a hole it was so bad. I got a used one for 50 bucks. Problem is I cant start it... will not stay running which, the starting thing was another problem I already had. I'll get back when fixed. My speedo was doing exactly the same thing. It was raining the night it started as well... the baox had moisture in it......

RT