> We have (most of installations) Firebird SuperServer 1.5.x Some

default

There is a known bug in FB 1.5 that may cause the delays. To some
extent it may help to use true local connection (not TCP localhost) or
disable multithreading or processor cores in Bios. Also, any database
activity run from a different connection usually "wakes up" the server
when it freezes. The bug was fixed in FB 2.0. See

http://tracker.firebirdsql.org/browse/CORE-419

Regards,
Michal