Its an old model, 1990 toyota avalon, loses some power in the rain,?

especially when driving up hills. Was noted that spark plugs were getting wet but how and how would a mechanic fix it? Change the wires? Hlep prevent wetness by putting in a new cap, rotor or plug wires? I am not smart re. car mechanics. And want to direct a mechanic in the best and cheapest (for me) direction. Thanks for all answers.