ATI Radeon HD 3870 has a clock-lock at 862MHz for the GPU. The RV670 GPU has multiple power phases internally, and PowerPlay is activated all the time. When you overclock the GPU, the PLL VCO divider will over volt the card just enough to cause a lockdown. The upgraded video BIOS available here will allow you to unlock the full potential of the card. You could tinker with the card to clock the RV670 chip all the way to 900MHz, massive 123MHz faster than the default clock.