I occasionally experience(d) the same kind of uneven development with my 'Blad negatives. Not often, but very annoying when it happens...
In those cases, the negatives were underexposed as well, so I assume that there is a correlation. What do you think? Is this possible?

I am using both a Paterson tank (500 ml) and Jobo 1520, and my developer is Spur HRX-III, which requires 2 initial inversions, then 1 inversion every 30 seconds, so not much movement here.
Film is Acros or Tri-X, sometimes/seldom Delta 400.