Truck has been running fine for quite some time and recently battery light comes on and starts to spudder and run really bad!! Stop and start again runs great. what could possibly be causing this, thought maybe bad battery,(its less than a month old) but it doesn't need a jump when starting, checked battery it was at 12.85 volts. could it be alternator? if so i would assume the battery would be dead when trying to restart. Any ideas would be greatly appreciated!