https://bugs.freedesktop.org/show_bug.cgi?id=72291
--- Comment #4 from Thomas Lindroth thomas.lindroth@gmail.com --- I also experience this problem on my HD5770 card. As far as I can remember it existed back when the UVD code was first released.
The freeze happens when a majority of the video windows is moved to the other screen. I guess it has something to do with vsync being changed to the other screen.
I also run git of all relevant packages except the kernel and I use up to date radeon-ucode.