So after 5 hrs, my "overclocked" cub has climbed to 33.9 gh/s (87.21% efficiency), while my standard/low clocked cube is operating at 31.6 gh/s (96.14% efficiency).

Any idea why it won't get anywhere near the 38 gh/s??  voltages all test out correctly too.  Can Cubes be too cold?  the outer case is cold, and the air exiting it is pretty cool as well (feeding it freezing air from a window).