Motorola V551 / V330 / V547 / V555 / V557

I'm having troubles charging my phone. When I plug it into the charger about half the time nothing happens (it doesn't begin charging). Other times it will begin charging, but then later shut off. When I come back to the phone it is turned off and has not been charged. It does this when I use both my charger and my wife's charger (also a V551). It also happened once with my wife's phone, but has not happened again. Any ideas what might be going on?

Thanks,

Scott