Sounds like your headlight switch may be on it's last legs. Turn the lights on, wait a few and feel around the switch. If the area is warm/hot to touch, REPLACE the switch.
Here's a link to a "FORD" page where the switch is used to run 2 relays instead of the headlights and all the others. I did this to my F-150 and the headlight voltage jumped from 10.9 to full battery voltage. I did this 3 years ago and haven't had a problem or replaced a bulb yet.
http://www.ford-trucks.com/article/i...r_Brights.html
This will definately help.
Larry