I would suggest checking the device for malware, viruses, and bad apps. Any of these could cause the problem.
You can also try starting up the phone in "Safe Mode" to see if it works fine in there. Granted you can't use many apps in that mode, but if the phone runs fine while in that mode, then you know there is something on it that is the cause, as mentioned above, which you would have to find and remove.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.