https://bugs.freedesktop.org/show_bug.cgi?id=82473
--- Comment #11 from dimangi42@gmail.com --- (In reply to comment #10)
Just to be clear is it a problem specifically with HDMI or using any display? Also make sure you fully uninstalled fglrx. It overwrites some of the libs on your system with it's own versions. You can also try disabling acceleration. Add: Option "NoAccel" "true" to the device section of your xorg.conf
It's a problem using any connector, VGA being another one I've tried. I did the installation by these instructions to the point, so I shouldn't have fglrx installed (http://forum.xbmc.org/showthread.php?tid=174854). I can try without acceleration if that's fruitful for debugging, but that's not a useful end result for me.