Not sure if this true for our cars, but this excerpt from Mad Electrical's tech page would answer the the original question. Of course the fix is as noted, relays.
Paul L.
Many owners of Ford cars and trucks, built in the '60's and '70's, especially with the four headlight systems, are familiar with the headlights flashing OFF and ON. (Like erratic Morse Code, after driving awhile on High Beams.) The problem occurred because the circuit breaker in the headlight switch was weak. Simply installing H-4's in most of our older cars and trucks will result with the same effect known to old Fords. (The circuit breakers in original factory headlight switches cannot handle the extra current draw of the more powerful headlights.)