Performance issue on Oracle 9i

We're sitting on really old hardware with a really old version of Oracle. There is a project to migrate to another application, but they estimate it will take another 18 months given the human resources that are available. That being said, this is a production financial application and is performing extremely poorly for queries against a specific table. It's not a huge table, 388,000 rows, but given the age of the hardware/software, I'm surprised we don't have more issues.

The query itself is poorly written, but I haven't figured out how to make it more efficient. I did manage to reduce the cost and execution time of the query by 50% by creating an index on the SEND_TO_DATE column--which can contain nulls--but it's still very slow. I also set the degree on the table to 8, which gave us a minor bump in performance. Any suggestions would be appreciated, specifically on how I can change the "not like" and "!=" predicates.

select *
from aps1.txn_proc t1
where t1.tx_ref not in

        (select t.tx_ref
         from aps1.txn_proc t
         where t.send_date is null
           and t.exp_cmnt not like 'PR%'
           and t.exp_cmnt != projno||tno
        )