2002 Pontiac Bonneville charging system

After replacing the battery and alternor earlier this year, my Bonneville alternator indicator will frequently read that its not charging. After driving it awhile (30 min. Or so) it will usually come back up to normal (about 14.) Occasionally, it will go back down, but once its up it usually stays. It almost always starts off in the non-charging mode. I've had the alternator and battery rechecked. No one can find a problem.
Any suggestions?
Thanks!