https://bugs.freedesktop.org/show_bug.cgi?id=36596
--- Comment #9 from Michel Dänzer michel@daenzer.net 2011-05-02 02:34:37 PDT --- Really sounds like the GPU IRQ doesn't work (reliably). Do the numbers on the radeon line in /proc/interrupts still increase when the problem occurs?
Maybe try some IRQ related kernel debugging options as applicable, to see if any of them works around the problem.