My truck starts and runs with the voltage sitting at 12 volts. When I am driving it slowly drops all the way to 10 volts, and stays there even when I come to a stop. If I have my running/ headlights on the dash lights flicker dim to bright and the voltage drops down to 9 volts. I have a new battery, the alternator is charging between 13.6 and 13.4. Any suggestions? I can't figure out what's wrong.