Hmmm... It sounds like the reticulation problem for this film won't be easy to ferret out. Do you know if Pan F is more sensitive to reticulation conditions than your regular 400 speed film?
Pan F is an older type emulsion, without some of the upgrades that made HP4 into HP5, then HP5 Plus (for instance). I recall it fogs much worse than most slow films, grain isn't as fine as even some modern 100 speed films, and its very possible the gelatin is softer than in more updated emulsions -- this latter quality, if correct, would make it prone to reticulation.