I'm having a similiar issue and hope to find answers too. I'm currently on version 188.8.131.52 from 8/30/2013. Whenever I try to install the updated version, my screen resolution becomes more blurry and not compatible with my monitor even though the resolution hasn't changed so I have to rollback to this version in order to keep my screen the way it is. Otherwise, the part of the screen isn't on the monitor and everything is blurry. I think it is also affecting the FPS at which I can run WoW at too.
Don't roll back, do a system restore from a previous date.
15.7 should work on your 6670 with win7. The clean uninstall might be helpful. USE DDU as mentioned a million times on here.