Keith, I worked on old cars like yours. I did repair on electrical problem. One common problem I always notice is the connector behind the fuse panel. There is one main feed that goes to the fuse panel through a connector. After years of usage the connector start to overheat and start to loose contact. It start to oxidize from the heat. Before you realize, you will have intermittent connection. The car just dies and one minute later it's alive.
My 69 chevelle did that.