In a vehicle that old it could be a lot of things. Here are a few to help get you started:
Assuming you've already eliminated the coil itself as a possibility and cleaned the battery terminals and ends, look into corrosion in or on any electrical connections, including inside the distributor. You should be particularly aware that ground wires are susceptible and should be considered suspect.
Battery cables should be replaced. The small end of these cables are very suspect as they are a simple "dry" crimp connection and exposed to the weather. Corrosion builds up inside the connector over the years, where it can't be seen and will eventually fail completely, but not after causing situations like the one you are describing.
Also check any other body to drivetrain grounds, and clean or replace as necessary. You may even want to add some additional ground leads. It never hurts to have good grounds.
Check the main, "whole car" fuse which is usually located in a fuse box under the hood. Corrosion builds up on these as well. You might want to replace it just as a precaution.
One or all of these are liable to be at the base of the problem.