Why does my cars headlights flicker when going down the road?

"pulsate" would be a better term than flicker. Intermittent problem, have changed out the battery and the battery terminal ends; problem still persists. A short somewhere, i'm assuming, but how can I track this down? This has to be annoying as heck to oncoming drivers, as I can see the lights going from dim to bright ( not high beams) on the dash panel as well as the path in front of the car. It flickers at about the same rate as a heartbeat.