https://bugs.freedesktop.org/show_bug.cgi?id=76564
--- Comment #56 from Christian König deathsimple@vodafone.de --- @Garrett:
That looks rather interesting. First of all please open up a new bug report, I want to separate this problem from the discussion here.
To this new bug report please add the output of "xrandr --verbose" and your dmesg logs of the 23.98 mode in the working and not working case.
I have a pretty good idea what's going wrong here, but you need to test a couple of patches to make sure.