I have a cube where one of the mini-blades does this also (OOOOOOOOXXXXXXXX). By restarting the cube through the web interface, I can sometimes get that card to display all O's, however one of the chips will read '999' and stay stuck at that value, and the entire cube will only run at about 10-15% efficiency and the hash rate suffers considerably, regardless of clock setting.

Ever seen that behavior before?