I don't know is my problem similar to this bug. Because the gap between transactions was about 300K and the sweep interval is the default one - 20K and in this database there is no way to generate big number of transactions - the users a very little and they do small number of operations daily. I've changed the sweep interval to 0, restarted Firebird and did backup/restore. Now everything works fine, there is no data loss, but this long sweep process was strange.