Distributor Cap & Rotor

Possibly a plug wire or cracked distributor cap. I'd take a spray bottle and while it's running (be careful not to get your body parts in danger of the fan) spray some water on the cap area. If everything seems fine, you could spray down each plug wire at a time. See if you find the culprit that way. For a vehicle so old, if you haven't done a tune up lately. It's probably a good time. Replace the distributor cap and rotor. Plugs and wires.

Check the battery.
When battery dies these problems can occur first check the battery is in good condition..

check if some vacuum hose at disstributor leaks or is broken.

Check for cracked rotor button or carbon missing from cap point inside where it touches the rotor . heck for carbon cracks and moisture in cap.

This problem usually happens when the distributor gets wet.You can either take it out and dry out off or just replace it as long as you have to remove it.Make sure you do both cap and rotor.Hope this helps.Good luck.