I posted a message some days ago relating to this as follows

Alert on Server01 at 13/02/2006 12:09:18

The processor is experiencing a low level of idle time. Consistently
low levels of idle time can cause performance problems.

Use Task Manager to view the top processes by CPU. If a service or
less important process appears to be unusual, try stopping and then
restarting it.

Firebird is the culprit.

Since then I have found some further information

In the Firebird log the following entries have appeared

Server01 (Server) Wed Feb 15 17:36:20 2006
Shutting down the Firebird service with 3 active connection
(s) to 2 database(s)

Server01 (Server) Wed Feb 15 17:36:20 2006
The database E:\DATA\BRANCH04.GDB was being accessed when the
server was shutdown

Server01 (Server) Wed Feb 15 17:36:20 2006
The database E:\DATA\SYSTEM.GDB was being accessed when the
server was shutdown

This appeared after I had shut down firebird normally. I know at the
time that none of the users was connected to the databases via any of
the programs, so something has clearly not disconnected properly from
the databases.

I have also seen this message
Server01 (Client) Wed Feb 15 17:39:19 2006
C:\Program Files\Firebird\bin\fbserver.exe: normal shutdown


Server01 (Client) Wed Feb 15 17:39:19 2006
Control services error 1061

Lastly, and more worryingly, this message has appeared

Server01 (Client) Fri Feb 17 03:55:56 2006
C:\Program Files\Firebird\bin\fbserver.exe: terminated
abnormally (4294967295)

I have validated the databases which contain no validation errors and
I have done a backup and restore on the databases. The problem
returned after this was done.

Using FB1.5 on Windows 2003