Based on the troubleshooting you've done, if the system restarts instead of shutting down, then, to begin troubleshooting here, I'd recommend the following, just to be 100%:
1. Restart the system into BIOS.
2. Once in the BIOS setup utility, press <F9> to restore BIOS default settings.
3. Reset any customized BIOS settings. Clear all DMI event logs by setting the option to "Enable", which is located in the Advanced/Event Log Configuration section of the BIOS Setup utility.
4. Press <F10> to save the new settings and reboot the system.
If the problem continues, I'd next need to know how your system's set up (basic info., only):
* Currently Running BIOS Version:
* Power Supply:
* Processor [please, include the exact model or sSpec#]:
* Memory module(s) [please, include the brand, size, frequency and part number of the memory module(s)]:
* Add-in card(s) (please, include the brand and model):