4 month-old D700, lots of the same issues recently with all 3 of my flashes (2 SB-900 and 1 SB-800). Nikon service center in Malaysia wants to say it's because of worn out contacts. But now I researched it seems to be the fit of the bracket?
I wish the SB800/900 had the old screw-locks like the SB-28's did. This seemed to prevent the wobble between the strobe and the camera that seems to slowly loosen the hotshoe...