DB2 ODBC inconsistent connectivity
I can't give you a definitive answer, but I would suspect slow authentication, both client and server being Windows-based: because of the client-server version mismatch there may be some extra authentication back-and-forth between them, and at some point either (or both) will traverse the Active Directory on the domain and possibly all trusted domains for user ID and group lookup.

Try setting the authentication method on the client to match that on the database server (SERVER would be the default), this might speed things up. For this you'll need to uncatalog and re-catalog the database in question.

Ask your sysadmins to check AD status if (when) you experience this issue again, may be there's a spike in load or some other issue.