I've been trying to overclock my (already) factory-overclocked ASUS GTX 670 today using Evga Precision X. I've been following this guide on overclock.net, and keep having an issue where the core clocks would return to the ASUS stock speeds, while running Unigine Heaven. While running the benchmark, the screen would temporarily turn black with the monitor's icon for 'input connected' present in the top left corner. The benchmark would continue at the a lower max frequency. Checking GPU-Z just shows that the boost clock is back to default. Precision X has no evidence of change, however changing the slider and hitting 'Apply' will work. The problem occurs while running Heaven for longer than a single benchmark, at 1140 MHz (offset +81MHz). The same problem occurs around halfway through the benchmark when at 1160MHz (offset +101MHz). The power target is at 117% and the core voltage is set to 1150mV. USing the fan curve found at the top of guide, temps have never gone over 60 C (140 F), and hang more around 50 C (122 F). I am yet to fully understand the effects of overclocking and behavior of graphics cards. Is this instability? Throttling? Is it caused by the power and voltage? What is it? What causes it? And, how can I fix or prevent it? Can someone please provide some insight into this.