The past few weeks, I've been watching my in dash volt meter real close. After my glow plugs kick off the truck will jump up to a normal charging range, but sometimes while driving it will be up around 14-16 volts and then idling at stop lights it drops down to 10-12 volts. What would be causing this? Its only spradic and its driving me nuts!!! Both batteries are good, I tested them both with the truck off and got over 12 volts. I'm runnin' outta ideas guys. Thanks for the help!