Apparently our index is hitting our soft memory limit for some reason, and without knowing what changed, this is a toughy.
The final version of this software is our now so I could just shut us down for a weekend and clean up our bulletholed database (it's been through the mill somewhat) and set us up all nice and lemony fresh. It would make us stable for a long time coming and should improve our performance.
If that is the case I would ask members to prune their inboxes if they are getting bloated, as that can shave off 5-10% of our database size. No panic yet though.