Scruff,
How are you rinsing and drying the film?
Is it possible there's reticulation from a temperature shock due to running water into the tank as your rinse method? Some of the earlier images looked a bit like reticulation but it's not easy for me to be positive.
Just to be sure, are you using one of the vacuum column 'archival' film reel washers fed by tap water? Or are you using the Ilford three step wash method, preferably with DW?
When you dry the film do you hang it at an angle so the water can run down one edge or straight down so it can run down the entire film surface and both edges?
Either way are you applying a weight at the free end to keep the film straightened out during the process? It can "pool" either way and change the apparent density of the edges, btdt.
Are you using a squeegee, your fingers, or a sponge to get most of the water off the film after rinsing prior to hanging to dry? I'm assuming you're not using a DSA/Senrac or the like for forced air drying while the film's still on the reel, right?
Got a ceiling fan running or are you near an air conditioning vent that might be changing the temp of the film while it's still wet? Long shot I know but it's worth asking I think.
Some emulsions are more prone to reticulation than others and it's not absolutely positive that the Arista is absolutely identical to Kodak Tri-X, at least not to me anyway for what that's worth. Apparent base+fog can increase if the film gets reticulated, maybe you could provide a magnified scan of some of the worse bits of film to see if there's a reticulated pattern present.
HTH,
Pete