I haven't seen it mentioned but what type of developing tank (steel, plastic neither..) are you using and what's your step by step process ? Aside from the possible double exposure (see foot reference) I wonder if this could be something happening between steps ?
If plastic, are you using that swirl stick ? Is there any chem leakage which could lead to light leak while processing ?
Another thing, is you look at the first link at frame 28A-29, the artifacts by the sprocket holes change as the frame # increases. No idea what this could mean though but maybe a clue of some sort ?