It's always happen when clocks changes. In this moment I observe a colored vertical lines during 1 second. When I change power method or power profile manually it's happens one time too. The lines like this: http://radeon.koolfy.be/wp-content/uploads/2011/03/garbage.jpg $ lspci | grep -i vga 01:00.0 VGA compatible controller: ATI Technologies Inc Madison [Mobility Radeon HD 5000 Series]
It happens to me too, and I think it happens to everyone. It is a very well known bug. As a temporary workaround I use the profile power_method with the low profile. It doesn't hurt the performance of my desktop even when using composinting and I can always set to "high" the card with a script when I need more power (like running heavy 3d programs). Anyway, I have a 5850 card, so people with a lower end card may not find the low profile suitable for them.