Mine used to do that when the battery was getting weak.

 

battery seems fine....needle hangs out at like 12-14v....?

--E

 

The voltage reading doesn't mean the battery is good. When the car is running, the gauge is reading the alternator not the battery. When a battery goes, it's the current output that falls off not the voltage. You need to have the battery load tested to tell if it is on the way out.