another simple thing to check is to see if the passenger side floor board is rusted through--causing moisture to get to the computer behind the passenger kick panel.

Also check all your spark plug wires, especially for arcing. Plug wires can short out when wet. The easy way to tell this is to first start the vehicle in a dark garage or at night outside and look for any light shows under the hood. If you don't see any, shut the engine off and use a spray bottle and lightly mist the plug wires, distributor cap, and coil wire, restart, and see if you see any light shows then.

Also check the electrical connections at the coil and make sure they are snug.

Are your front wheel well splash guards in place? How about the front skid plate (if '86 or newer) or the splash pan if '85 and older?