1984 f250

my headlights have started to flicker on and off for about 20 seconds at a time. all other lights are fine. only head lights. im thinking its the switch. i want your thoughts before i go ahead and replace the switch

 

The switch has an internal circuit breaker. High resistance in the headlight wiring and harness, connections. If you clean up the circuit path the problem may go away.

If you want to see why, place a voltmeter probe on the negative battery terminal and the other on the negative terminal of the headlight bulb. Turn the headlights on. The voltage drop will be shown as a positive number. Anything more than a few tenths of a volt is excessive. This heats up the switch causing it to trip. Check the positive side too.

 

Does it do it on both high and low beam? It could be a bad connection at the dimmer switch. Floor mount dimmer switches are exposed to moisture and grime which can cause corrosion in the connections.

 

Switches are a common failure point. For a permanent fix install relays along with a new switch, it'll make your headlights appreciably brighter too.