I had mottled highlights some time ago, too, but for a surprizing reason: When
working with polyester supports on white polyester final support, during the
"waiting" time before development the hardening chromic compounds seemed to
diffuse. We found that the very short soaking by just pouring a bit water over
the tissue was the reason. By lengthening the soaking in cold water to 1-1.5 min
and heavily moving the tissue before combining it with the final support solved
that problem. But as I don't know how you work, you may have a totally different
problem.
Klaus Pollmeier