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Does your Sierra still have the factory spark plug wires, distributer cap and rotor on it or has it been a long time since they have been replaced?
Alot of times when you develope a miss only during wet times it is a result of the plug wires getting wet and allowing spark to jump to ground(ie: the engine). Take a spray bottle filled with water and mist over the spark plug wires along the entire length and see if it begins to miss again. If so it is time for a tune up. Plugs, wires, cap, rotor,filters(air,fuel) and a PCV valve. It would be a good time to clean the throttle bore at this time to. Should take care of your missfiring and improve fuel milage.
Try wetting the wires anyway to see if the missing starts. There is still a chance the wires are bad. Pull the cap off and check the contacts inside for discoloration. I see many of these GM caps on the 5.7 have moisture problems turning the inside contacts a greenish blue color. Moisture builds up in the cap from plugged distributer vents in the bottom of the distributer. It definately is a moisture problem in the ignition system or possibly at the PCM connector.