I don't think it could be a diopte problem, because the diopter will only have effect on the viewing of your view finder screen.
You cannot change the focus of a projected image (as on your focus screen) by putting on glasses. - Hope that makes sense to you.
I first would try to reproduce the error in a controllable environment - studio, home etc.. if it persists, check if they are always off the same amount.
Are no parts of your out of focus pictures in focus or are they in focus maybe on one side yes and on the other no?
(Did you drop the camera or did it get hit recently?)
Most probably it is going to be a collimation issue (as has been suggested).
If setting your diopter is difficult with a picture on the focusing screen you might as well take your lens off, point the camera to a bright source and focus your diopter to see the grain of the focus screen. It dosn't have anything to do with the "incoming" picture, it just makes it easier for your eyes IF you usually wear glasses for sight.
Good luck to you.