Try starting up the tablet in "Safe Mode". If it starts and stays running in "Safe Mode" then there is something on the tablet that is causing the problem. Bad app, bad update, malware or virus. You would need to determine the cause and then remove it.
NOTE: In "Safe Mode" most apps won't work, you should, however, be able to keep the tablet on without it trying to restart constantly. This is for testing only.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
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.