Samsung Galaxy Note 4 stuck in reboot loop - what is the problem? Here is the situation: My phone (Samsung Galaxy Note 4) turns on and works perfectly fine if I am do not click on any app. 

Then say I click on some app. If I try to press too many things in too short an interval of time, then my phone restarts itself. Also if I clicked to turn on data or WiFi, then the phone reboots regardless of whether I select an app or not. When the reboot is complete, it displays a much lower battery level than it had when it turned off. Then it reboots again, and continues a few times until the battery hits 0%. Then I need to charge it up.

(*) if my phone was plugged into a charger, then this does not happen - it never reboots, and never has any issues. But if I keep it plugged into a charger all day, it effectively is no longer a smart phone, but is more of a desktop PC

Questions: 

What is wrong with this phone? How can I find out what is wrong? Can it be fixed? If so, how?

Note: I am a novice, I don't know much about technology other than how to use my device for day to day stuff. It is not rooted or anything like that.
Update: I went to a Samsung repair centre where they concluded that this was a hardware issue, and the hardware needed to be replaced. (The cost of this was £200, so it does not represent value for money in my opinion)