Regarding charging issues, I am 99% sure that the problem lies in the microusb port: loose connection to the board.
Best way to diagnose is running Galaxy Charging Current app. If you spring to the paid for app (it's like 50pence!) you get auto refresh. Take your cable and plug in putting pressure at different angles. If you twist plug while in socket you may sustain a maximum charge of 900, but upon moving to a different position the charge will gradually drop in 100 increments. Seems the sammy roms are little less fussy in regards to power stability as opposed to cyanogen/aosp
Unfortunately unlike the S2 and S4 models the USB port is not available as a modular replaceable component so it will need resoldering. Mine's in warranty so shall be getting a repair done rather than attempting to do myself.