https://bugs.freedesktop.org/show_bug.cgi?id=87796
--- Comment #14 from Clésio Luiz magolobel@yahoo.com.br --- I tested now with 2 displays, the Benq monitor (DVI cable) at 120Hz and a Lg TV (HDMI cable) and the glitches disappear.
When I disable the TV (from System Settings) and disconnect the cable, the glitches are still gone. Only when I restart the system the glitches come back.
I should mention that any refresh rate above 60Hz triggers the problem, not only 120Hz.