The "WARNING: inbound connection timed out (ORA-3136)" in the alert log indicates that the client was not able to complete it's authentication within the SQLNET.INBOUND_CONNECT_TIMEOUT time.(Defaults to 60 secs after 10.2)
Take look of the following
1. Check whether local connection on the database server is successful & quick.
2. If local connections are quick ,then check for underlying network delay with the help of your network administrator.
3. Check whether your Database performance has degraded in anyway.
4. Check alert log for any critical errors for eg, ORA-600 or ORA-7445 and get them resolved first.
These critical errors might have triggered the slowness of the database server.