https://bugs.freedesktop.org/show_bug.cgi?id=87796
--- Comment #15 from Alex Deucher alexdeucher@gmail.com --- Created attachment 112228 --> https://bugs.freedesktop.org/attachment.cgi?id=112228&action=edit add some debugging output
Based on the timing for the 1920x1080@120 timing from your xorg log, it looks like it should be just under the switch limit. Can you apply the attached patch and switch to the problematic mode and then attach your dmesg output so I can see what's happening?