First of all: excuse my bad english.

I have the same problem with my HD 7950; using hangouts, playing Left for Dead
2, or watching a flash video my screen goes crazy with vertical lines or grey
fog. Started when i upgraded to testing repo (Archlinux) and downloaded the
newest linux-firmware package, who includes TAHITI_mc2.bin. I suffered this bug
on kernels 3.14 and 3.15. For now, i am using 3.15.1 kernel, and the old Tahiti
firmware, and it seems stable.