Connection problem non-standard port Microsoft, Sql Server, 2005, Named Instances

Hi,

We have a server running SQL 2000 & 2005.  The 2005 server has named instances, running on various ports.

If we try to connect to one of the named instances (using query analyzer for example), the connection fails saying the server is unavilable or does not exist.

If we go into the Sql Client Network Utility (or modify the advanced connection properties in an ODBC connection to turn off 'dynamically determine port') and manually set it to the correct port, the connection works.  That's fine, and makes sense.

If we then set client network utility or the odbc connection back to their previous settings, the connection still works.

Can anyone explain this behaviour?  Why do the client machines 'remember' the connection details for subsequent attempts, yet fail to work the first time?

We are trying to find a way to make every connection work first time without having to go through the fudgey process of connecting once manually.

Thanks

Mick