I also thought that DBI was causing the uneven development, so I did some sheets by time & temperature and got the same problem. To rule out uneven staining, I processed on sheet in a tray by just rocking the tray. Same stuff. I am inclined toward the presoak theory at present. I just processed 7 sheets of 8x20. Handling the film was very slow so the first sheets into the presoak were in there a fairly long time. Now I can't say for sure which sheets were which, but some of the films came out perfect and others had varying amounts of mottling. It was as if the motting was inversely proportional to the time spent in the presoak. I will pursue this further. I will also start using metaborate in my presoak again.