Comment # 2 on bug 909089 from Egbert Eich

I'd strongly assume that this is something in your desktop session that's
constantly retrying to (re)set the same mode.

To find out what it is you'd have to 
- install gdb, xorg-x11-server-debuginfo, xf96-video-intel-debuginfo and 
  xf96-video-intel-debugsource, 
- log in from remote from another machine(!),
- attach gdb to the pid of the Xserver: 'gdb --pid <PID> /usr/bin/Xorg, 
- set a breakpoint to the function that prints the offending message:
  (gdb) break sna_crtc_set_mode_major
- let the Xserver continue to run:
  (gdb) continue

Once you've hit the breakpoint, 
 - generate a backtrace:
   (gdb) backtrace
 - identify the X request that's triggering it: a function in the backtrace
   starting with 'Proc' and a single argument 'client'.
 - go up to this stack level:
   (gdb) up <level>
 - Inspect the client argument:
   (gdb) p *client->clientIds
The name of the client should be contained in the data structure displayed.

If this doesn't work, please post the backtrace.
Thanks!