It sounds like an indexing problem. Have you tried saving a compacted copy of the database?
No not yet, is that something that does occur intermittently ?
I'll try that and get back to you.
It can. Depends on how much, and how, the database is used. Circumstances that contribute to index corruption include large-scale delete / import operations, modifying database schema while the database is in use, and, especially, peer-to-peer sharing.
But it's a good idea periodically to make a compacted copy, or even a clone (and re-import all the data). Contributes to stability.