Unfortunately, deactivating ColorTiling and/or ColorTiling2D has no effect. I
also tried to set "SwapbuffersWait" to "False". 

Actually, as far as I remember, I tried to test most of the options available
(http://manpages.debian.org/cgi-bin/man.cgi?sektion=4&query=radeon) two years
ago, when I installed the board. Then, I switched to fglrx-driver.

I think it’s not related, but… often, X failed to restart from the virtual
console (weird pixelized screen or black screen, or nothing at all).
I had to reboot the computer to double check the result (and then, I did not
see any error in the Xorg log). 

Thanks again,
Benjamin.