It seems as if this problem is widespread. Do you think KnC has manufactured numerous cubes and shipped 3 winners and 1 loser to each of their customers?
I have only one cube out of four capable of occasionally reaching 75 MH/s. Mine was shipped the first day of shipping I believe. Maybe they knowingly shipped defective cubes to meet September deadline, and started shipping good cubes later, putting only one bad cube in each shipment hoping that people wouldn't complain.