https://bugs.freedesktop.org/show_bug.cgi?id=33790
--- Comment #5 from Alex Deucher agd5f@yahoo.com 2011-08-05 10:45:00 PDT --- vblank interrupts (or interrupts in general) may not be working on your system. Try `cat /proc/interrupts` and see if the count on the radeon line increases. If not, you might try booting with pci=nomsi or noapic or irqpoll on the kernel commandline in grub.