I have the 5-reel tank, and use ID-11 (same D-76) 1+1 or 1+3. I do 4 slow inversions once a minute. No sign of uneven development so far, touch wood. I wait for the "glugging" to stop before doing the second half of the inversion. That doesn't happen quickly, so possibly your inversions are too rapid to completely mix the developer. Just a thought.