I have very similar problem. And people said it can be solved by
replacing battery. I ordered replacement battery from
Taobao (Mine is Nova) and you should be able to find the replacement battery from Aliexpress.
Also I heard that the battery last longer if you don't use Google Play. I didn't test it myself but I never enable Google Play (still have the shut down problem). It seems to me that the root cause is the stock battery.