A defective charger would be most likely the cause of your problem. It was for me,
when I experienced this (both times for my 770 and N80)
ended up buying a new one (AC-4E). Both chargers showed the same
fault, the wiring broke near the charger, the strain relief (the bendy bit)
doesn't do its job properly. You should be able to buy one for less than
15 Euros (~ $20 atm?). If you have any soldering skills and time to spare you might be able to fix it also.

Bill