K5 partial out-of-focus problem

With my K5 + DA* 16-50 I get a falling away of focus on the right side. This is not a depth of field problem, as subject detail the same distance from the lens on the left and middle of the frame is fine. Indeed, it doesn`t seem to be an issue with subjects close to the lens, but only when focus is set at, or close to, infinity. I have the autofocus set to centre, I don`t think this is the issue. Is the sensor somehow slightly out of alignment? Any thoughts welcome.

Sounds more likely to be the lens. If it's a function of focus it sounds like one of the elements is going out of alignment as you focus further away. Try another zoom and see if it happens with just the 16-50.