Moto Z Droid shuts down randomly with lots of battery left

[JMU13]

For about the past week, I think after updating the system to 8.0, my Moto Z Droid shuts down most of the time when I try to use it. As soon as I plug it in, it shows that it has plenty of battery left, and immediately I can turn it back on. But it won't turn on unless it's plugged in. If I unplug it and try to do anything it shuts down again, sometimes right away and sometimes after several hours, even though it has plenty of battery power. I have a Tumi power pack backup battery that attaches to the back, but that makes no difference. It still shuts down, even when the Tumi is fully charged. I saw others complaining about a similar problem with the new operating system. What's going on?? And more importantly, how do I fix it??