I'm afraid this sounds like my problem. It happened to me when I was on holiday and was unable to hook my phone up and properly diagnose. However, over the course of a week my Note 4 went from being a little twitchy to complete and total unusable brick. The only thing that kept it going for most the week was an app called "Wake lock- powermanager" that prevents the device going to sleep. If you read the reviews on that app page you will see that we are not alone. In fact there is currently a class action lawsuit in Canada against Samsung because it is claimed they have used dodgy eMMC storage chip for the on board drive which is causing all of the issues.
The only thing that apparently will fix the problem is replacing the motherboard. Or buying a new phone. Which I did.
Sorry to be the bearer of bad tidings.