Verify there are no unfilled gaps in the system where a peripheral or blanking plate should be. If there are any gaps then please replace the devices or plates in order to ensure correct airflow.
Ensure the PC is off.
Disconnect all external cables including the power cable.
Hold down the power button, so the inner circuits are closed and any remaining power (flea power) may dissipate. Hold the power button down for 5 seconds.
Now reconnect the power cable and start the PC.
Is the PC shutting down or overheating when left in the BIOS? You can get to the BIOS by pressing the F2key at POST - when the system starts up, but hasn't started Windows yet.
If not, download and update the BIOS with latest available version.

If this stops the issue then your problem is resolved.
If this doesn't resolve your problem proceed