Battery dissapearing/reapearing with an X in the battery symbol

Hi all,

I recently replaced RAM, Hard Drive and Battery in my MacBook Pro (late 2011) model. I'm now having issues where the battery disconnects and reconnects at random times. I can see this happening as the battery turns into a battery with an X inside, and then back to a regular battery charging symbol. When the computer is put to sleep by shutting the lid, sometimes it just won't wake no matter what combination of keys and power button I try. Occasionally the cursor gets hijacked, for lack of a better word. It becomes jittery and jumps around the screen.

When I opened up the machine there was a screw missing from the original battery (there was only one of the two screws in place). Can I get replacement screws? Is that (a loose battery) likely the cause of my issues?

I've done a PRAM/NVRAM and SMC reset, but that hasn't seemed to do anything. I've also cleaned the track pad after some advice from the internet saying that might have been an issue.