Are you using an Agression level of 11 or 12?

Secondly try to reduce as much graphical rendering on the desktop as possible (e.g. turn off Aero if your using Win7 or Vista, turn off widgets etc.)

My cards run at 99% at agression 11 or 97% at agression 7.  I have 2x 5830's and 2x 5850's in another PC under Win7.