Yes, I'm trying to decide between a faulty battery and a faulty connector. It's the inconsistency that's confusing - sometimes it works, but often not. That kind of makes me lean towards a connector problem, in that a failed battery would probably fail consistently to charge.
I've just found a suggestion that cache issues can cause slow charging and high battery usage. To be fair, that was referring to Android 4.4, and I'm on 7.1, but it's an easy thing to try, so I have.