Comment 6 for bug 274234

I'm not sure 472252 is related, since that involves a configuration where both -ati and -radeonhd are used simultaneously, which is quite different from your config.

The next step for you is to collect a full backtrace. Directions for doing this are available at http://wiki.ubuntu.com/X/Backtracing. That should give us a clearer view into where it's failing.