@DGM, @Konvictus177, I cannot see diagonal banding. The arrows added to th image point to a vertical line, not a diagnonal feature.
If I had unwanted parallel diagonal fluctuations across the entire image, I would do a 2D FFT, then zero out the unwanted frequencies on the appropriate diagonal in the frequency domain, then do the inverse 2D FFT.
If I had a single unwanted parallel diagonal fluctuation going across the image, I would try to fix it in the spatial domain. I would set up a for loop to proceed along the cenerline of the diagonal, replacing each point with the median of itself and the N points in a perpendicular direction on either side. Then I woudl repeat for the parallel set of points on either side of the centerline, etc. There are variaiton so on this approach that you could try, such as replacing 3 or more perpendicular-to-the-diagonal points at a time with the median of those 3 or more points.