why is my headlights intermittently-flickering after about 20 minute.very scary. on 1990 Ford Mustang GT

when i drive at night i have to hurry home before dark.i have no diagnostic code but i did change my headlights to a one pieces sets and use the new wire that came with the sets.never had no trouble until i made the change.

Asked by for the 1990 Ford Mustang GT