How to fix my head lights?

ok so i drive a 1974 dodge dart and for some reason after my head lights are on for about 20 minutes they start turning on and off at random intervals. Everything else in my car electrically works fine its just the head lights. And only when i turn my lights off when im sitting behind someone at a red light do the lights stay on just long enough to make it to the next light. Any info would be an amazing help because i drive 40 minutes home after practice. :/