Paul,

It seems we have the same common problem. I also had precisely and clearly delimited zones in even parts of the negative, skies, water... obviously more visible on the neg's side. As said above, it has been immediately solved using a longer presoaking (3-5 minutes instead of 2 previously), and adding some Na carbonate - about 2 g, I don't precisely weigh - or metaborate in the presoak water, according to my friend's advices.

From time to time, I always got very very light streaks, but I think it's because of agitation, it should be easy to set.