Replacement Battery Acting Odd

My 5S battery was acting up, had to be kept plugged in all the time for it to work at all. I replaced it with one I got off ebay. Most of the time the battery works fine, but sometimes, might not happen for 2 days, might happen 3 times in one day, the phone will shut itself off and won't turn on no matter what I do. If I take the phone apart and disconnect and then reconnect the battery it will work fine again. The battery itself isn't running out of juice because when I turn it back on it will often have 80+% battery left. It seems to work fine the rest of the time, though I have noticed that it does run a little hot at times. It doesn't have to be hot for this to happen, it can just be sitting on the table not being used and when i pick it up I will see that it has died and will have to take the phone apart to disconnect and reconnect the battery. Also when I open it the ribbon cable seems firmly attached to the connector, I have to pry it off, I have tried pressing down to see if it's loose and it doesn't seem to be.