https://bugs.freedesktop.org/show_bug.cgi?id=66963
--- Comment #103 from Alex Deucher agd5f@yahoo.com --- (In reply to comment #102)
If you could explain me where is kernel command line (when I'm seeing GRUB menu push "e" or "c"; and if "c" what to do after I entered radeon.modeset=0 in there?), I would try it. I'm just a newbie using kubuntu a few months.
in the grub menu select the kernel you want to boot and press 'e' then move to the end of the line that starts: linux /boot/<blah blah blah> and append the options to the end of that line. e.g., linux /boot/<blah blah blah> radeon.modeset=0 1 The '1' means boot into single user mode rather than X.
Also, if you haven't already, try disabling aspm: linux /boot/<blah blah blah> radeon.aspm=0 radeon.dpm=1