Digital BW, The Print

I have a print I am making that has a very empty sky with about a 50% gray tone that fades darker to lighter. When I print this I get vertical streaking in the area at the top of the print and continuing down about 2". The streaking is not fine like a clogged nozzle, it is a very light banding that fades in and out gradually with a peak to peak width of about 3/4". It is not highly noticeable, but there if you are looking for it. 

The lower part of the image is busy, so I flipped the image 180% in photoshop and printed it again thinking that if it is a printer issue on the left side of the printer it might disappear. In the flipped image, the streaking is much lighter, but still there. I cannot see this issue anywhere else in the image other than in the top of the sky where the tone is very even. 

I also think this defect fades as the print dries, but copy that is a month old still has some of the streaking, so it does not completely go away. 

Any ideas what causes this?