HELP!! Headlights keep shutting off!

Hi.
I have a 1991 Sierra, and all of a sudden my headlights have been shutting off while I am driving, I have found that if I turn on my high beams and them turn them off the lights come back on but only for a little while and then they shutt off again, it can be any where from 10 seconds to 5 min. but they always shut off again.
Please help, I drive at night a lot and constantly flashing my headlights is getting annoying

thanks in advance

I don't know if our GMC trucks have this but I figure they do since it was the common way to do it.
There usually is/was a self resetting circuit breaker instead of a fuse in the headlight circuit.
So if a wire to the headlights get shorted somewhere, it will trip then cool off and reset. So you might have a wire rubbed bare up there.
My daughter has an early Bronco and the headlights were doing this, We found by tapping the headlight switch they would cut out and come back on. We replaced the switch and found the automatic breaker was part of the switch, and it was coming loose.
Later this AM I'll check the shop manual to check this and post if I find more.
Bob B

I have replaced the head light dimmer switch and no more problems!!

thanks for the help