Thank you for your post.
When you exit a record, the information is written to disk. Also, if you leave your computer while editing a field, all other previous edits in that record will write to disk after a few seconds of no activity.
FileMaker Pro should not freeze, so it is possible your database file may be damaged. You may want to consider running a Recover (File menu) on the database file. FileMaker will do its best to fix the file. Try using the Recovered file and see if you continue to "freeze".
Do any other applications "freeze" since you moved to Snow Leopard?