Maybe the voltage regulator turning on and off. I've seen cars do this when the battery is very low on water. Check the battery to see if it is full of water. It may say "Maintenance Free" but if the caps are removable then pop them off and look. If it is full you might go to an auto parts and have the battery tested for free. If it is good battery>>>>>>>>>>>>>>
you'rs most likely need an alternator. the regulator is inside the alt. depend on the model of your vehicle. If the charging is bad buying a new one will solve your issue with the voltage regulator. 100 percent positive on this. good luck