After coming to a stop cars RPM's drop below 1,000 and sputters

Okay, car starts up fine an sits there idling fine, once I pull out go down the road come to a stop cars rpm's drop below 1,000 and sputters. Then it starts working its way back up to the 1,000 rpm's. Also noticed at night headlights go real dim at red light give gas they get brighter. I was thinking alternator.