https://bugs.freedesktop.org/show_bug.cgi?id=84292
--- Comment #2 from José Suárez j.suarez.agapito@gmail.com --- Those cards are r600g, not radeonsi. In general terms radeonsi is Radeon HD 7XXX and newer.
I am using radeonsi mesa git (both with llvm-3.5 and llvm-3.6) with linux 3.17 rc5 and I am not experiencing that problem. In fact, I have been able to play for 3+ hours non-stop without any problem.