Ford has that problem in alot of cars... replace the headlight switch.
you will find that when you run the car with HI-Beams on it happens quicker.
With the LO-Beams on...longer time before it starts to cut in and out.
Over the next week or so it will get worse... start cutting out quicker and more often.
the part takes a couple minutes to replace...
1. with a pic, release the lock spring and pull the knob off
2. 2 screws holding the instrument cluster cover on
3. unscrew the big nut holding the switch in place.
4. remove the wiring harness
5. press the springed button and pull out the switch rod
6. Re-install, reverse order
I did it 3 days ago while the wife ran into Wal-Mart for 10 minutes...she never even knew I did it! Simple