I have had this D700 for 6 years. Now it is clipping the top of all my images. please see the image below
Interesting... Try taking the lens off the body and taking a time exposure. Look into the camera body through the lens mount. See if the sensor is 'clear' with nothing projecting in front of it. Since the issue is at the top of the frame the blockage will be at the bottom of the sensor. I think the autofocus system is mounted in the bottom of the mirror box, and there may be something going on there... If you -do- see something, you can try to gently nudge it to see if it unsticks, but if it were mine I'd probably just send it back to Nikon for repairs.
I don't know, but my guess is that the mirror is not quite raising all the way. What else could it be? You can remove the lens, and watch the mirror as you press the shutter button, and probably can see it not quite raising all the way. Sounds like a repair is necessary.