I have been testing Pyrocat HD and MC over the last several months with 35, MF, and 4x5. The attached images show something that happens only on 120 film, probably more often than I notice, since it takes a flat gray area to see it. It has happened with HD and MC. Looks like telephone wires across the top of the image. See the long scaled up image for a closer look.

If you look closely, you can also see, in the upper left corner, an area of slightly weaker development in the sky.

This is a rollei neg, so this defect occurs at the top of the frame, close to the space between frames, not at the edge of the film.

The film is FP4. The developer was Pyrocat MC 1.5:1:100 at 72F. I use a single 120 reel Nikkor tank, with enough developer to cover the reel by 5-10cm, but leave enough room for the developer to move around (420cc). I presoak for 4 minutes, agitate for 2 minutes, and, on this particular test, with MC, developed for 18 minutes, agitating once again for 20 seconds at the 10 minute point, halfway between the end of the 2 minute agitation and the end. All agitation is a twisted inversion, slow roll of the wrist, then rolling in the sink left to right. Slow but steady movement.

Stop is a minute of running water, and TF4 fixer.

This happened at least once before, with Tri-x and HD.

Any ideas?