OCCT is a stability checking tool that was created back in 2003, and was regularly updated since. It enables you to fully test your computer's stability, using 4 different tests : CPU:OCCT and CPU:Linpack aimed at testing the CPU, GPU:3D for the GPU stability, and Power Supply, a combination of CPU:Linpack and GPU:3D to fully load your Power Supply.
Each test can be run indefinitly, or for a fixed amount of time. You can even include an idle period at the beginning or at the end of a test, to better see how your computer's temperature rise from an IDLE state to fully loaded, and the other way round.
OCCT also embeds a monitoring engine, giving you complete readings of the computers inner temperatures, voltages, and fan speed, and displays them both in graphs plotted in real-time and at the ne of a test. If you are worried about performance with the real-time graphs, don't : even if they are really light, just hiding the graph panel will stop them from refreshing.
OCCT even protects your computer from overheating or damage by including alarms you can customize for every sensor : once you go above (or below) the alarm value for a temperature, voltage, or fan speed, the test will stop to protect your computer.