Unpredictable sorting slowness, goes away on restart

The setting:

FMS 5.5 on Windows

FMP 5, 5.5, 6 clients for 5-10 users on Windows

Server and all clients appear to have adequate memory; network performance is strong.

The problem:

Sometimes, when a user views a layout that includes a portal showing join records in a file with a large number of records, FileMaker pops a "Sorting..." progress dialog, that counts down from a number in the thousands, then goes away. This is not surprising in some cases, since the portal shows records in a particular sort order, and there are a lot of join records, and over a network these things can take time.

Here's the mystery:

It doesn't happen all the time or for all users!

Here's the pattern I've figured out so far:

1. A user can launch the database and browse records for hours without seeing the sorting dialog (or having to wait for it to finish).

2. After editing a record or creating a new join record, the sorting slowdown begins... even on the same records and layouts that had better performance in 1. above.

3. Another user can be looking at the same records on the same layouts at the same time, and not experience the sorting slowdowns.

4. Any user who's started experiencing the sorting slowdowns (which become chronic and sap performance enough to be a serious problem) can make them go away by quitting FileMaker and opening the database again.

My client and I have been experimenting, yet we can't find a way to avoid these chronic "Sorting..." dialogs, aside from restarting FileMaker.

Is this problem familiar to anyone? Does anyone know a work-around whereby we could avoid the chronic sorting and attendant dialogs?

Thanks in advance,

Allen