Hi,
did you try a different charger?
Sometimes their output, especially the cheaper ones, is not constant and causes charging problems.
EDIT:
it could also be that your micro usb connector is getting loose. @Stefan has experiences to get a new one.
As @Lidwien said: the battery guide has good information about FP batteries:
Good luck
Werner
P.S. I found out that I can save a lot of battery if I turn off GPS on my FP1!