Sometimes it is not the Kindle at fault but the quality of the charger/lead/connection. Non-OEM stuff often just isn't very accurate.
As an alternative to Amazons own charger, I recommend trying the OEM Blackberry PSM04R mini-usb charger 5V 700mA. It does not seem to suffer from the same intermittent loose connection problems of some other chargers; it always makes a firm connection