A workaround in to disable KMS at boot: I added radeon.modeset=0 in the kernel options of the grub configuration and the problem seemed to disappear.
Here is the related line in /boot/grub/grub.cfg:
linux /boot/vmlinuz-2.6.32-21-generic root=UUID=2aa5a050-dd81-4fca-83ba-c269a8dbdc96 ro quiet splash radeon.modeset=0
A workaround in to disable KMS at boot: I added radeon.modeset=0 in the kernel options of the grub configuration and the problem seemed to disappear. grub.cfg: 2.6.32- 21-generic root=UUID= 2aa5a050- dd81-4fca- 83ba-c269a8dbdc 96 ro quiet splash radeon.modeset=0
Here is the related line in /boot/grub/
linux /boot/vmlinuz-