This script is one of our favorites. It finds the top N sessions that have been affected by a particular type of resource wait, and enables event 10046, level 8 in those sessions for the specified period. This event captures every wait event and its wait parameters, and writes the information to the process trace file together with the normal sql_trace output. The wait parameters can then be analyzed to understand exactly what is causing the performance problem. 

declare
  type sid_type is table of number index by binary_integer;
  type serial_type is table of number index by binary_integer;
  cursor waiters is
    select
      e.sid,
      s.serial#
    from
      sys.v_$session_event  e,
      sys.v_$session  s
    where
      e.event = '&Wait' and
      s.sid = e.sid
    order by
      total_waits desc;
  sid sid_type;
  serial serial_type;
  n binary_integer := 0;
begin
  open waiters;
  loop
    n := n + 1;
    exit when n = &Sessions + 1;
    fetch waiters into sid(n), serial(n);
    exit when waiters%notfound;  
    sys.dbms_system.set_ev(sid(n), serial(n), 10046, 8, '');
  end loop;
  if n > 1 then
    sys.dbms_lock.sleep(&Sleep);
    loop
      n := n - 1;
      exit when n = 0;
      sys.dbms_system.set_ev(sid(n), serial(n), 10046, 0, '');
    end loop;
  else
    sys.dbms_output.put_line('No sessions to trace.');
  end if;
end;