I concur with the advice to ensure cooler air is flushed through the fan. I have several cubes and at room temperature they were operating at around 35 G/H each while overclocked... but would reboot every few hours into normal clocked mode.

With cold air (around 30-40 degrees) they are all operating at a consistent 38 G/H while overclocked without auto rebooting. It makes a big difference. Pray for more polar vortex weather.