I think I'm hitting a similar issue on a RV770 (Radeon HD4850) hardware, while
playing the game Left 4 Dead 2.

I was on Fedora 19 (kernel 3.14) and upgraded to Fedora 21 (kernel 3.17). Since
this time, the game became unplayable due to GPU lockup. See the dmesg in
attachement.

Here is the summary of the tests I then performed on a fresh Fedora 20 install
:
kernel 3.11 + mesa 10.5-devel + sb = OK
kernel 3.17 + mesa 10.5-devel + sb = BAD
kernel 3.17 + mesa 10.5-devel + nosb = OK
kernel 3.17 + mesa 10.3.3 + sb = BAD (worse, 3 times more lockup)

I'm not skilled to compile/install a custom kernel, so I will just try to
download a 3.16 kernel from koji when the site will be up again, in order to
see if the issue appeared with 3.17.