I had the exact same issue with my AMD Radeon HD 6850. Turned out it was due to some settings I had changed in AMD Vision Center in order to play Dark Souls. I was overriding the application's settings and Anisotropic Filtering was what was causing the issue. Once I had that turned back to application controlled the graphics were back to normal. If you're still having the problem, try creating a profile for dontstarve_steam.exe with everything set as application controlled.