Comment # 23 on bug 43829 from Dennis Nezic

I had a similar problem with my x700 (rv410) radeon, and fixed it with:
  radeontool light off 
  radeonreg regset 0x00000284 0x1bd34208 
  radeonreg regset 0x000002d0 0x003c00a1 
  radeonreg regset CL:03 0x001c0007 
  radeonreg regset CL:04 0x0002004a 
  radeonreg regset CL:05 0x0002004a 
  radeonreg regset CL:06 0x0002004a 
  radeontool light on

Have you guys tried the radeontool light thingy?