[Zope] Connection Problem with Oracle

I created a connection to an Oracle database. Frequently, the connection breaks and I got a database error in the page that displays the query result. When I logon as a manager I find that the connection is closed (by itself, nothing changed in Oracle or Zope), when I try to open it I get an error "Invalid connection string". Even if I create another connection it can't work.. After a while it works. Even after restarting the Zope server the problem persists. Thanks Mirette