Light Artifacts with ND 3.0-1000X

Question

I recently purchased a B+W ND 3.0-1000X to mount on my Pentax K20D for taking extended time-exposure shots of various subjects. I've enclosed an image from a recent shoot. Almost all shots have strange banding artifacts with exposures around 30 seconds. It is most pronounced in vertical images but is also present in horizontal photos as well. I do not believe that there is a problem with the B+W filter, but thought one of your photography experts might know what is going on. There was fairly strong side-lighting and if I had been able to see these artifacts on my LCD, I would have tried to block some of that light. Any thoughts?

Jeff Clay
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