What lens are you using? Does the problem occur on all of the lens, or one in particular? I had an 80-400mm VR lens that wouldn't focus even though the sensor light told me that I was in perfect focus. Also, since I'm old and feeble, I have invested in VR lenses as money become available.
Another thing to check is the camera set-up, "portrait" settings will/should give you a softer image, but then again that's not blurry as you describe. There are some very good DVDs available on camera set-up for the D200. They have been a tremendous help to me in the past.