The "problems" when focusing (back and forth) I also partially. It always occurs when the depth of field is very low. So telephoto and maximum aperture. For me perfectly understandable if the photo does not know what he is focusing. I think the photo is responsible and not the objective.
Unsharp my images are also partially. But only with larger exposures. I think you are on the safe side, if you try the time out of your hands to hold smaller 1/300.