Sanyo SCP-3100 Cellular Phone "cell keeps starting shuting" Questions

...KEEPS DYING MY MOTOROLA RAZR JUST STARTED SHUTING ITSELF OFF WHEN I AM TALKING. THE BATTERY GUAGE SHOWS A FULL CHARGE. Try a new battery,if that does not work the phone that have the phone

Cell is shuting off after about 3 mins of talking on it! It is a sprint cell phone its a sanyo,I just took it to a sprint store and they said it needed updated so they updated it and they said ...