It sounds to me like a text book example of spark plug wire failure. A
high moisture content in the air can make old plug wires fail because
water and or water vapor passes through the insulation that has gone bad
from age and use and shorts the connection to the plugs. Change out all
of the secondary electrical system components such as the spark plugs,
spark plug wires, including the coil wire to the distributor,
distributor cap and rotor and you have a very good chance of fixing this
problem. Good luck!