The switches have contacts in them with bimetal strips that will open up when they get too hot, like a circuit breaker. You either have a bad switch or excessive resistance in the circuit somewhere. Find your battery wire to the switch and connect the voltmeter to it with the connector plugged in. Connect the other end to the hot wire at the headlamp, turn the headlights on, you should be reading around 0.1 volt. If it's more than that you have excessive resistance in the circuit. Turn the lamps off, disconnect the head lights and switch and start probing backwards, measuring the resistance to outside and inside of bulkhead and back to the switch. It should be less than 0.2 ohms. If it is measure the resistance of the ground circuits from the headlamp connector to the body ground and back to the battery to find out where the excessive resistance is. If all this fails to find anything go get another switch.