Automatic Windows Restarts is likely due to having "automatic restart" set for when Windows crashes (BSOD).
To turn this off:
- Go to Start -> Control Panel -> System.
- Go to Advanced.
- Under the Startup and Recovery section, click Settings...
- Under System Failure un-check "Automatically restart"
- Hit "OK" to save and exit.
Next time it happens you will be able to see what the issue was.
Alternatively, you can look at your Event logs: Start/Event Viewer