Comment # 38 on bug 82201 from Sebastian Parborg

Kai, I have an 290x and I'm having the same problem as you.

However I do not have windows installed at all. So I think we can rule that one
out.

For me it seem like the card loses the ability to reclock after a while.
However I have regained the reclocking ability by rebooting to use fgrlx and
then reboot back to use radeon...

I'm just as confused as you are why it stops working. :S