There have been instances of 9i version of oracle masking the actual wait events with the "null event". You can maybe enable event 10046 for the session that's experiecing the "null event" and see if the event can trace the actual wait events that might be occuring. To enable event 10046 for a different session, you can use the dbms_system.set_ev procedure:

SQL> exec dbms_system.set_ev(sid,serial#,EV=>10046,LE=>12,NM=>'')

Good luck!

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