i had something similar happen to me on fomapan but for 120mm i don't know if it applies... and once with fuji acros 100 (120 again) i think... If i remember correctly it happened when i did vigorous shaking the first 30 sec or so. In reading about this problem... others mentioned fomapan doing this over several blogs... They also mentioned the stop bath possibly being a problem.. but it's not so... cause i have never used anything other than water. I think perhaps it is air bubbles... I now use the turning rod provided with the patterson tanks. I haven't encountered this problem since.
Also if you wet the film before developer- i no longer do this (fuji says its unnecessary)- this might also contribute to it (the formation of 'spotting) for some reason, maybe saturating the film with water making it harder for the developer to take effect in some areas.