HELP PLEASE. Water Makes 02 Silverado Run Rough

We have an 02 Silverado 6 cyl with about 90k miles. When we drive through a good sized puddle, and especially when we went through an automatic car wash with the undercarriage wash, it starts running really rough. It loses all power, bucks and shakes and tries to die. But after it dries out about an hour later, it runs fine. We have replaced the distributer cap and rotor and fuel filter recently, but it still does this every time water gets up under the truck. What else should I be checking? Thanks for your help.