Very fine grain can take on a brownish tint, because of light diffraction. The same effect is apparent in warm-tone papers and developers. This might contribute to the effect.
Dichroic fog is not so visible on a light table, but when you look at the neg at a pointed angle. It is mainly caused be silver re-disposition during development (in solvent-type developers), and not much affected by fixation (because it is already metallic).
If it is a fixation problem: I would use continuous agitiation at least for the first minute, not just a few inversions every 30sec. 120 is wider then 135, and so less exchange of fixer is happening within the spiral.