@ Ed,
I knew it was about category indexing! Your data tables aren't talking.
Is this a continuation of the upgrades initiated a few months ago? If it is, what's the ETA? December??? LOL! I'm teasing! Chances are a variable is missing from a master table...or you're using text rather than number indexes and somebody made a typo and can't find it. I don't envy your coder.
Could you keep us posted on any changes to the system that may impact category indexing? If we know when the system is being changed, we can check our pages and let you know if models are missing.