Comment # 17 on bug 65963 from Maciej

Ok, it was fine during around 7 or 8 hours of usage (mostly browsing in Chrome,
some gaming, a tv show with smplayer and vdpau). However today it started
happening again.

To clarify, my system is (fresh installation made yesterday):

Ubuntu 14.04 64bit
Kernel 3.16-RC7 from mainline repo
Mesa git from Oibaf PPA
HD7770 1GB

Switching to kernel 3.15.7 doesn't help.