Hmmm; usually when shooting portraits you would focus on the part of the subject, which needs to be sharp – in this case on the closest eye. I dont think there is a lot of contrast on the nose or between the eyes.
I would first clean the contacts on all the lenses and on the camera. A microfiber clothe and a drop or two of Deoxit Gold would do the trick.
The next step would be to make some tests in good light with good focusing target at distance 50x focal lenght with camera on tripod. Could be that the camera just needs some AF focus adjust.
I would also try some other lenses if they give me the same problem. Could be the lens, could be the camera or could be combination of specific lens and the camera.
Still can't figure it out why on earth would you need a screwdirver set, but I have a feeling you won't take into account my advices so I have no idea, why I bother writting this.