iphone losing charge while "charging"

Occasionally my iPhone will say "this cable is a third party accessory" and will not charge for about an hour. After an hour it'll begin to charge normally. Now though it will charge to 6% and without me touching it, it will deplete to 0%. It's plugged into my wall and I've tried other cords and blocks but still nothing works. I've had many problems with this phone and genius haven't helped me whatsoever. Please help me. Its an iPhone 5c.

Thank You