Comment # 11 on bug 87796 from Clésio Luiz

(In reply to Alex Deucher from comment #10)
> It looks like maybe the dpm memory reclocking occasionally misses the
> vblank.  Do you still get the glitches if you connect a secondary display or
> disable dynamic clocking?  To disable able dynamic clocking (as root):
> echo high > /sys/class/drm/card0/device/power_dpm_force_performance_level
> to re-enable it (as root):
> echo auto > /sys/class/drm/card0/device/power_dpm_force_performance_level

The High value makes the glitches go away. As soon as I set the Auto value, the
glitches reappear.

I don't have access to a second display to test right now, only tomorrow.