A stress tester that targets cpu and ram. http://www.ocbase.com/perestroika_en/index.php?Download
You need to monitor your cpu temps while its running with HWmonitor, Realtemp (Intel).
You may need to re-apply the thermal paste on your cpu or your gpu might be heating up.
Check around the fan inlet and outlets for a build up of dust and fluff blocking airflow.
Download and run 3d Mark to test your gpu and ram also along with the HDD test to eliminate that.