I was going to suggest checking the connection as soon as I read your first post, but it seems you're gotten it figured it now! I recently replaced my battery too and had the exact same issue, reboots and shutdowns. I suspected it was an issue with the connection because the plug kept wanting to pop out of its socket during the install. Took it apart and that was exactly it. The battery I bought was a genuine Nokia, and get this: the manufacturing date was 2015! I couldn't believe it. I'm on my N9 as I write this, but when I get to a PC next I'll share the link in case anyone wants to buy a reliable new battery!