Run a stress test software such as Prime95. Let it run for about 10 minutes, this should usually be enough time to make one of your core fail. If this happens, chances are you're running short on CPU voltage. Which then it would be necessary to increase the voltage by the next smallest increment.
Usually when you increase CPU speed and notice that your system is unstable, you need more volts. However do not go above 1.4V. Above that and it can be dangerous. Stay between 1.35-1.41 if possible for high overclocks.