You are welcome :-)
If there is no /etc/modprobe.d/radeon.conf , create it with the content
lembas told you:
$ sudo sh -c 'echo "options radeon modeset=0" > /etc/modprobe.d/radeon.conf'
"And reboot to see if that alone helps."
If not, lembas meant you to edit only the "Device" section of
/etc/X11/xorg.conf with those contents. You can probably find a
/etc/X11/xorg.conf~ file or something if you edited the first with emacs or
nano (I don't know about others): it is a backup of the edited file, which
you can restore (mv xorg.conf~ xorg.conf) and then edit properly.
If what lembas recommended doesn't work for you, I would recommend you to try
installing the linux-generic-lts-belenos (which installs Linux-Libre 3.5) and
testing that. If it doesn't work, try more recent Linux-libre versions:
$ sudo add-apt-repository ppa:linux-libre/ppa
$ sudo apt-get update
$ sudo apt-get install linux-libre
$ sudo apt-get dist-upgrade
(these are the instructions at
https://launchpad.net/~linux-libre/+archive/ppa )
OR
follow the instructions at http://jxself.org/linux-libre (another source of
more up-to-date Linux-libre pre-compiled binaries for Debian-based systems --
that PPA is for Ubuntu IIRC). [I can't access jxself.org right now, get no
answer.]
