What causes Wavy lines in Betacam SP footage over black areas?

Hi, I have 2 SONY 537 cameas shooting BetacamSP that I just had checked out with a technician who said they are both adjusted correctly. I have a set consisting of 2 director chairs with a black muslin back drop. I am only using Flourescent lights at 5100 degress K to lite the set. Both cameras are recording a set of wavy lines over the black areas of the backdrop. On one camera records almost a perfect picture with only barely noticeable wavy lines over the black areas and the other camera with the exact same settings records an image where the wavy lines over the black areas are very noticeable. I thought it might be the flourescent lights but I took both cameras outside and videotape an old sign that has white lettering on a black background, and I still am getting the wavy lines over the black but not as noticeable outdoors in sunlight. Does anybody know how to fix this problem? Desperate!

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