Yan,
Your problem looks to me to an issue with your final wash/rinse. As you say that its occurred on different formats and different sizes in Ilford's range, I can't see the materials being at fault. Apart from all else, I suspect that a lot of other people with film from those batches would be hopping up and down as well! I occasionally used to suffer from marks on negatives. I live in a very soft water area and concluded that I was overdoing the wetting agent in the final rinse. I now use literally only one drop of wetting agent in a full tank of water, then tip that away and have a "final final" rinse in just plain water. No problems since I started doing that.
Simon's kind offer of advice and investigation is typical of the service we enjoy from Ilford (other firms, not just photographic, should take note!) but somehow I think you're wasting your hard-earned money posting your negs to England!
Hope you get to the bottom of the problem, whatever it is!
Steve