So it turns out that my issue is down to the cube.  I just received a second cube today.  Hooked it up to the same PSU, same switch, and pointed it to the same proxy and it hashes on High.  My first cube did for 24 hours, but will only hash on Low now.  Any ideas as to what might have happened and if it's fixable?

Still, an average of 35GH/s between the two isn't that bad.