Comment # 56 on bug 76564 from Christian König

@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.