Ive had this problem with older trucks with the dimmer switch on the floor. It was usually a bad dimmer switch. But...
If the lights come back on when you switch from bright to dim you have a bad dimmer switch.
If they go off after you have been running the high beams for a while and you have to wait a while for your headlights to work at all start with your fuse box. If the headlights use a thermal fuse switch it out with a regular fuse of equal value. If this fixes the problem you need a new thermal fuse.
If it blows the fuse check your headlight wiring for shorts and make sure you have good connections at the headlights, headlight switch and also make sure that the fuse plugs into the fuse box tightly.
Look in the relay center under the hood and see if you have any headlight fuses or relays in there. Make sure the connections are tight and if you have a headlight relay try switching it with another one in the box.
If you have installed any aftermarket headlights that draw more current than the factory bulbs try putting the factory bulbs back in.
If you still haven't solved the problem you probably have a bad headlight switch. The high amperage high beams may be heating up the switch to the point you lose connection.
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2002 6-speed, 5 inch straight pipe, Icebox, Edge Drag Comp/Smarty. Isspro pyrometer, mechanical boost guage & electric fuel pressure gauge. FASS 95, Valair kevlar/ceramic clutch...