I don't shoot with my D1x much. I bought it used just to mess around with, so I have probably only taken 250 photos or so with it. I've noticed this distinct horizontal noise/grain pattern in several photos under different conditions. It is visible on the body of the speedlight, and in the background of the lens shot.
My instinct is its just the noise of the sensor, and it got amplified in post processing? I did do some batch stuff in LR, including tweaking the exposure. Does this seem like something different to anyone else?