Do you have the latest firmware for the D70? I've seen astro cameras do this when they use the wrong firmware. Also, some cards are not approved by Nikon for the D70 so you might get some beef about that too if your cards, aren't.
And a wild guess. I looked at your sample image and maybe this is a mechanical/timing issue. It could be that some times the mirror is not coming up all the way and it's blocking some of the light. I see no evidence of pixel corruption other than a very straight line with an exposure difference. The image projected on the sensor is upside down so the darker part is on top, where the mirror goes to. Sort of like a bad flash sync.
Just a guess.
Also, if the problem is intermitent Nikon might not be able to reproduce it. Make sure you include sample images.
. I know better 
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