sounds like a bad connection at the headlight switch to me. or where the headlight wires connect to the headlights may be dirty.

 

I had the same problem in my '69. I chased it for ages. It turned out to be the hi / lo dip switch. The low beam runs through the switch and when it gets old or tired, the contacts go intermittent. Try tapping the hi / lo switch with your foot when the lights fail and see if they come back on. I got my money on it.
cheers