Are your machines plugged directly into an outlet or are they plugged into a power strip or something like that?

I had this exact same problem with my 6950. I had my computer plugged into a power strip with a bunch of other things; like monitors, game consoles, tv, speakers, etc. Plugged into the strip, I was getting the exact same results as you. But when I plugged the computer into an outlet all on its own I managed to get it to 420 kh/s with an intensity of 18 and thread concurrency of 6010.