(In reply to comment #3)
> > Does this happen with particular pictures or randomly?
> 
> I haven't tried to reproduce it yet, I will try to find the time for it.
> 
> > It might be worth getting the PITCAIRN_mc2.bin microcode and trying a 3.15 kernel.
> 
> I guess once I've managed to reproduce it, otherwise we don't know if it
> fixes anything.
> 
> PITCAIRN_mc2.bin will be included with the kernel 3.15 firmware package
> right?

It's available in the linuxfirmware tree:
http://git.kernel.org/cgit/linux/kernel/git/firmware/linux-firmware.git/commit/?id=4848a1e059de5aa723ff6627b90de8a5d9632626