73 Valiant Intermittent Headlight Issue

I would start with the light switch harness connector terminals as they can cause HEAT when poor, and heat up the breaker, or the breaker might be getting weak--replace the switch
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There are several important junction points in no particular order of appearance

LOOSE / poor terminal connections cause HEAT which can cause the switch breaker to overheat and TRIP

1...Power for only the headlights (not park, tail, or dome) comes from the ammeter circuit welded splice. RARELY that weld can fail and cause trouble

2...That power goes to B1 at the headlight switch, and THAT TERMINAL can be bad/ loose/ corroded

3...The internal switch breaker can be getting weak and do what you describe

4...The output lead for the headlight power at the switch, that terminal can also be bad/ corroded, and trip the breaker

5...Not breaker related, but the terminals at the floor dimmer switch can be poor, or the contacts in the switch itself. After all, they are down on the floor

6...Next, the hi and the lo beam power goes out through terminals in the bulkhead connector which can give trouble

7...At some point the lo/ hi power wires get out to the headlight where they are branched to left and to right, and of course the lamp connectors can be old/ corroded

8...Many? most? these girls have a common ground eyelet out in the harness which of course must have a good clean solid ground