Stain problem with Paterson Multireel 8

I have recently started using my Paterson Multireel 8 Tank for developing 3 or 4 120 reels. The problem is that I have started to notice some strange round stain on some part of the negatives as you can see below. To me they seem to be bubbles, but it's something that I have never experienced before with my smaller Paterson tank. As you can see the film is Tmax 400 developed with D76 1+1 (500ml of solution for each roll); I always do 1 minut of initial continuous overturning, then 4 overturnings each minute with heavy slamming of the tank on the table afterwards. I really can't figure out what's the problem since my process did not change. Furthermore the spirals are tighly fixed with a rubber band so that they can't rise up during the process. The only thing I have noticed is that the problem always occurs on the film placed at the bottom of the tank and never in the one at the top and always on the first frames (the ones on the inner part of the spiral)
Any one else has experienced this? Any idea on how to solve this? Thanks a lot for any help