Setting maximum HDMI output framerate

[greenwal]

Thank you for xbian, it is an amazing piece of software... I have it running sweetly, but with one catch. Due to the layout of my house, the HDMI cable is quite long, and suffers from interference, but only when the framerate is 50Hz or higher.

I am familiar with setting hdmi_modes in the config.txt. If I use auto, the Pi output is always 50Hz (as the TV correctly reports it supports it) but that is flaky.
If I set config.txt to force a 24hz mode, then films run fine (as they are 24hz), but TV shows suffer horribly as they are 25Hz (or other). Switching causes the opposite problem.

Is there a way I can ask xbian (or the Pi), "use auto framerate, but only 30Hz or lower"? I suspect I must edit my own EDID file, but that looks like a rocky road...

Thank you!

Software
XBian version: 1.0 Beta 1.1
Overclock settings: None
Hardware
RPi model (model B 512mb):
SD card size and make/type: 16gig class 10
Network (wireless or LAN): Either
Connected devices (TV, USB, network storage, etc.): Panasonic Viera