Phone not charging properly

So a few weeks ago my Xperia started to have strange symptoms and shutting down suddenly. Battery level would show around 97% or so and then I'd put it down for a minute, it'd say 50% and then shut down right there. So I went and replaced the battery in it, and then also replaced the charging port. It's acting a little better and I can usually get through my day without having it tethered to a battery pack, but it's still doing it. I'll eventually pick up my phone and it'll say the charge is around 50% and then shut down. Also when this happens I'll put it on the charger and it stays at 0% charge for a long time before finally starting to show a charge on it. It works fine connected to a charger or a battery pack.

I'm hoping I'm not going to have to replace the motherboard. That's the only thing I can thing of that I haven't replaced yet that's related to this problem. Any thoughts?