It sounds a bit like "bromide streaks". The cause is often incorrect agitation. It can be made worse by the wrong developer for the film. Old, concentrated soups (like DK-60a) with modern, thin emulsion films seem more prone to it. Sometimes the mechanical construction of the developing tank influences it, too. In any case, agitation technique is very critical with today's films. Read the instructions, and practice helps. That said, it happens to all of us (whether or not we admit it) sometimes. If you had digital darkroom equipment (scanner, computer), Photoshop can often do miracals for this sort of thing, but not usually without some hard, careful work.