+1 on dana's suggestion. As the gap between the ignition rotor and wires widens, the voltage required to fire the plug goes up. If there's moisture providing even a high-resistance path to ground, it may still be less than the now-increased resistance between the tip and terminal.
If the engine is running like (bear with me on this, this is really the best way to describe it) harrumph-harrumph-harrumph, it's probably misfiring on one cylinder, pointing to the distributor and everything downstream. If it's rough and struggling, but evenly so (doesn't struggle cyclically), it's probably upstream of the distributor. Either way, rotors and wires go fast in these cars and should probably be replaced.