Headlights going on and off

The headlights (both) will go off for a few seconds. This does not happen all the time. Sometimes this will happen once or twice during a long drive. Other times, it will go off and on several times within a span of 5 minutes. The headlights are the only lights that go off. All other lights remain on. When the headlights go off, it will turn back on when I switch from high to low beam or low to high. I thought it might be a short so I took it to a auto repair shop and the mechanic said he tested the wiring for a short and did not find any. Has anyone else experienced this problem? Thank you.

This is on a 1993 F150 4X4.