Check the points and condenser, and under the advance plate where they mount, and if there is a bell shaped shield over the points, look inside it, too. If there is a shield, any water that gets under the advance plate will condense there when the engine warms up, drip on the points and condenser, and get slung around by the cam, shorting them out, but it may not condense or be evident in the cap or rotor. Voice of exasperated experience.

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