It must be an electrical issue. The governor pressure solenoid and sensor is one possibility as mentioned above. But it should not be effected by puddles. The wiring from the solenoid exits the tranny on the top through a connector. My guess would be that you have a problem at that connector, perhaps the wiring is damaged, salty water affects the signals... Hard to tell.
1997 JGC Laredo 4.0 4x4 99900mi * 0W30 German Castrol since 75k * 225/75/R15 Firestone Destination AT on Ravine rims * winter 235/75/R15 Bridgestone Blizzak DM-V1 * UC suspension * Monroe Reflex Monotube shocks * factory skids * KOR steering brace * rear Addco SB * Magnaflow cat and cat back system * complete 5.9 interior