I would ASSUME it's the battery, that is, if it didn't happen BEFORE you bought the new battery. Did you keep the packaging of the battery in case you may have to exchange it? If not, I would just toss it(which would be a waste unless you can exchange it)and buy a new one to see if it happens again. And if THAT'S not the case, then that would be where Apix or Brandon would come in where it could be the player itself. Good luck donn and let us know how everything goes, kay?

Where did you get the new battery? Maybe it was one that sat on a shelf for some time? I would check with the place you got it from.

Tired of waiting for your device to do what you need it to?

The problem with firmware glitches are, that while some can be reproduced, and have a negative effect right away, some issues arise over time or not at all. Almost like a hybrid virus would react (don't worry that's not what it is lol just an example). But i've seen where the issue thats a known firmware problem, will effect random players that are all the same. (As i've tested multiple players with the exact content, firmware and batch and not all have the same problem).
Firmware is indeed a strange beast at times, but good to hear all is well now :)