Try starting it up in "Safe Mode" and see if it reboots then. You won't be able to run most apps in there, but if it doesn't turn off, then you know it is something on the tablet. Bad app, virus, malware, etc., that you wil have to remove to resolve the problem. If it happens in "Safe Mode" as well, then it is more likely a hardware issue.
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...