Electrical Demon?

Failing ballast resistor, failing ignition switch, etc.., etc...
You could throw a lot of parts at it. In some cases that is the best route since you're replacing 40 year old parts that are destine to fail eventually anyway.
If you want to find and fix only what's wrong today, First take a close look at connectors. A white connector in the engine harness near the right valve cover is one of the usual suspects. The harness connector, also white, from the ignition switch, under dash, is another. If there is no visual evidence found,
Get a clear understanding of what voltages you should have where and when, then start diagnosing.