When testing cards at below -64°C the NVIDIA driver gets confused and thinks the GPU overheats which causes a loss in performance. When started with /fixcoldbug the attached GPU-Z build will increase the temperature reading that the NVIDIA driver sees by 65°C (it does not actually increase GPU temperature). This change should result in stable benching at lower temperatures. Please note that a warm reboot might not remove the change, powering off the system and back on will. See also here: http://www.xtremesystems.org/forums/showthread.php?t=250634 Please report your success in this thread and here.