1. Back up your current bios using GPU-z (free)
2. Open your bios in RBE (radeon bios editor, free)
3.go to clock setting page and set your GPU, RAM, and voltage. Each clock number represents idle, 2d< 3d, etc. You want them all the same so make the all the same.
4. Save as a NEW bios, do not overwrite your original. Matter of fact make a copy of original and hide it away in another folder
5. Save modified bios
5. Use ATI winflash (free) to flash the new bios onto the card
I used this workflow to change the 2D clocks on my 5770 because once you overclock it the 2D settings default to "Idle" making multimonitors flicker really badly.
And and FWIW there is a small chance you could permanently kill your card. New cards have multible bios that make this a lot less scary. I had no issues experimenting with this several dozen times but YMMV Good luck