Hi Hans,
so the radeon driver seems to work but maybe needs some tweaking.
1) Let's have a look at Xorg.0.log.
grep -i chipset /var/log/Xorg.0.log
grep -i render /var/log/Xorg.0.log
(Maybe, append the file to your message)
2) Is there a file /etc/X11/xorg.conf?
3) Take a look at the man-page of "radeon" (man radeon)
Regards,
jvp.