It's caused by the multiple readout amplifiers in the sensor not being properly calibrated together, so that every eighth row of pixels has a different overall exposure level than the rest. I see a bit of this effect in underexposed ISO 1600 images on my 1D-MkII occasionally, but Neat Image seems to get rid of most of it. If the sample was pushed more than a stop, I'd say the easiest fix is to learn how to expose properly. But if that is a RAW conversion with exposure set at 0, then you'll need to have the camera sent in for sensor recalibration or replacement.