pulsating headlights at idle / 66 valiant

HEY guys,
Just finished installing a new heater core, 1st time out at night and at idle, she pulsates thru the headlights/ lights brighten then dim/...kick up the throttle, same ****, but turn a signal switch on and it stops...makes me think maybe a voltage regulator, the later is possibly an alternator needs changing, what do you think..?????
1966 Plymouth valiant signet / SLANT 6 :help: