1 - Verify that your listener is running (lsnrctl stat).
2 - Are your using MTS (shared servers)? If so, turn it off, unless you have
a good reason:
http://www.dba-oracle.com/t_mts_multithrea...vers_shared.htm
3 - Check that "DIRECT_HANDOFF_TTC_listener_name"=ON in listener.ora.