Help me slant six dan

First you need to determine if the headlights are REALLY burning out or if this is a power supply, that is, harness problem

A IF the headlights are actually burning out, then you likely have a problem allowing system voltage to climb to dangerous levels, IE a charging problem.

Measure the battery voltage while running, wiggle connectors, especially the bulkhead connector, and see if the voltage goes up

B If the headlights are simply stopping working, and not actually burning out, you need to go through the harness step by step and find the problem. END TO END

Start with the headlights and the connectors. Are the connectors clean and tight? There is a ground(s) out at the headlights for the harness, and it can easily fail --rust, etc

Get a meter and test light. You have a shop manual? A wiring diagram?

Look at the simplified diagram in the MAD article down the page:

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

See where it says "welded splice?" This is a big splice under the dash a few inches down the black ammeter wire. IT feeds off several places, including HEADLIGHT POWER to the headlight switch.

Pull the switch out and inspect the switch/ and connector for heat damage, corrosion, tightness. Power comes out of the switch and goes down to the dimmer switch. The switch itself can be "dodgy" or the connector at the switch can be loose/ corroded. From the switch, high and low beam power goes out through the bulkhead connector WHICH CAN HAVE loose/ damaged/ corroded terminals, and feeds off to the headlights.

You need to find out WHERE you are losing this power

WHEN THE HIGH BEAMS quit, do you have a high beam indicator on the dash? This is IMPORTANT. If you have no headlights, but the high beam indicator WORDS, then it means everything is OK up to the headlight switch and dimmer switch, and that the trouble is either at the bulkhead connector, or out in the engine bay.