I have a number of roll with the same history that are in canisters only and a lot more in their boxes and cellophane bricks of 10, all frozen. Hopefully the bricks have had less chance for moisture penetration - assuming thats part of the cause. Given this result was a little intermittent between the rolls, I am going to run a number of canister-only rolls of HP5, Delta 400 and FP4 through the camera with the lens cap on then develop them as a bit of a test on the remaining stock.