https://bugs.freedesktop.org/show_bug.cgi?id=93826
--- Comment #52 from iuno@posteo.net --- It could be fixed by increasing the default display clock: https://people.freedesktop.org/~cbrill/dri-log/index.php?channel=radeon&...
@Eike: you have a faulty monitor, please don't suggest people here to send theirs back because there certainly *is* a software issue.
@Jonas: Did you try forcing the mclock to high? It doesn't require a higher core dpm state but should fix your problem.