I suspect your shutter is the issue as it is an arc that follows part of the swing of the shutter mechanism...

Part of this video is the movement of the shutter at 1/10 of a second and makes it easy to see what is going on.
http://www.youtube.com/watch?v=tU-ZHadg3Jo

I am wondering if there is a gap of some sort allowing more light through at higher speeds, therefore overexposing that part of the image, causing your problem.

It might be there at lower speeds but more subtle. At any rate, Nikon needs to fix it. Good luck.

David

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