Van runs poorly in rain and wet roads

Chevy 1 ton cargo work van. Drove van about 40 miles in rain and wet roads. Vehicle ran fine. When I got back in and started the van, the RPMs would spike and then drop. This was continuous. I put the van in drive and pressed on the gas and it would barely move, maybe about 15 miles per hour. I turned the vehicle off and restarted it. The same result. I tried dropping from over drive to 3rd. This seemed to make a difference and got me back to my shop. The van drove fine in over drive for about a week until it rained again. The Chevy dealer cant seem to find a problem even after running it through a car wash to simulate rain or wet pavement. This has repeated itself over and over again. I assume something is getting wet while driving on wet roads, just not sure what it could be.