I have the camera one of the later models and its fine, are you shooting JPG? also I noticed I want shoot lower than F18 and the image becomes softer due to Interpolation, but that's general, I will say this, what software are you using to work on your images, I had noticed when this camera came out, the images were terribly soft, but that was due to the software rendering the 5D3 images badly, Im happy with my photos and been a terrible pixel peeper, just make sure you have the latest firmware, all software packages are up too date, if it still persists, it could be a problem with earlier body's, but I'm not certain on that
Louis