Years ago (like 20 years ago) I had a 1983 Ford Escort that did the exact same thing. Eventually found out the negative battery cable was just a little loose and was losing continuity with the the vibration of the car on the highway. Gas station attendant found it for me. I felt like a complete moron.
Its not the most scientific approach in the world, but its an easy check that won’t take but a minute to rule out.