Overheating device with random apps spiking CPU usage
A couple of things: 


Applications that show running processes are only able to show the user processes (meaning no kernel processes)
Android gives more priority to the application in the foreground (the one you are currently seeing). When you open the the task manager, the other processes are being capped. The measuring you are trying to do influences the results.


If you can provide more details by measuring externally with one of these:


Try installing Android Studio:
Post the Log Cat results: 
Or use the Android Debug Bridge (adb) with adb logcat – you can view your Android device’s log.


With one of these you can actually profile your running Android phone over a USB connection.

I don't think anyone can give you a complete direct answer without some more information, except if maybe they had the exact same model number and issue.