https://bugs.freedesktop.org/show_bug.cgi?id=87796

--- Comment #10 from Alex Deucher <alexdeucher@gmail.com> ---
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

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