Comment # 10 on bug 87796 from Alex Deucher

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