My guess would be a failing switch. If the contacts are getting hot the switch may be thermo-cycling where the contact gets so hot it pulls away, cools, reconnects, gets hot, pulls away, cools, reconnects, so on and so forth.

The switch itself may have a thermal cutout built-in for this heat stress issue. They are not fused (usually) and it may have been their way of keeping the switch from just melting down. Would have to open one up. I have about five or six used '65 switches. May just pry one open and see what's inside one of them. It'll be close to yours in design.

Of course, I don't know any of this for sure. Just 'pothesisizing.