ORA-12547: TNS:lost contact
 *Cause: Partner has unexpectedly gone away, usually during process
 startup.
 *Action: Investigate partner application for abnormal termination. On an
 Interchange, this can happen if the machine is overloaded.

ORA-12535: TNS:operation timed out
 *Cause: The requested operation could not be completed within the time out
 period.
 *Action: Look at the documentation on the secondary errors for possible
 remedy. See SQLNET.LOG to find secondary error if not provided explicitly.
 Turn on tracing to gather more information.

ORA-03136: inbound connection timed out
 *Cause:  Inbound connection was timed out by the server because
          user authentication was not completed within the given time
          specified by SQLNET.INBOUND_CONNECT_TIMEOUT or its default value
 *Action: 1) Check SQL*NET and RDBMS log for trace of suspicious connections.
          2) Configure SQL*NET with a proper inbound connect timeout value
             if necessary.

Yes, can do a tnsping and hostname

ping <the server in this entry>
tnsping <the entry name>
sqlplus xxx/yyy@<the entry name>