there is lots of articles on the web that describe inbound connection timed out (ora-3136) in the realms of oracle 10gR2 and the new default setting of its sqlnet.ora-parameter SQLNET.INBOUND_CONNECT_TIMEOUT (the tnsnames.ora-parameter INBOUND_CONNECT_TIMEOUT_listener alike). people propose to reset the parameter back to 0, from 60, seconds to achive an indefinite wait for login authentication against the database. so far, so bad. that is, just tackling problems by turning around control buttons will not solve the problem nor will it help in understanding the root cause of the problem. this is especially true, iff the problem appears spuriously, not in a determinable order or user/application segment.
but, what may be the root cause? many argue that network misdesign along firewalls or nat might provoke the inbound connection timed out (ora-3136) warning with the alert.log. hhm, do not know, never saw or experienced that. however, what i can show is that this very warning can be easily induced by connecting a privileged session to an idle instance (remember the difference between instance and database, especially on windows; an instance is represented by a service and the database will be bound to it during startup) and such a way prove, that also an database/host overload can be responsible. just change the missing or unbound database with a non-responding database in your mind to grasp the experiment approach.