Well just for the sake of experimenting, and I know this sounds like a radical idea , but how about turning off AF and trying to manually focus some shots. Maybe try some with IS turned both off and on. And make sure the diopter setting for the viewfinder is set properly for your eyes. Unless your camera and subject are completely immobilized, maybe close down 1/2 or full stop to give you a bit of a "cushion" in your DOF.
Good thoughts. I will give those a try.