Are you sure that the charger and the batter that you are using with the device is working perfectly? Because there could be so many reason that could cause such problem with the device. but still it would be better to determine first whether there is some problem with the deviceís firmware or with the hardware? Have you ever tested the device by using another battery or the charger if not then you should first try to test it and then observe the result. In fact, if you are getting result with variation that simply means surely some problem is with your device.