optimizer (9i) solaris

Summary of situation:

- select statement runnnig fast/ok on 8.0.5/Sun 220R
- upgrade app to 9i/Sun v480...screams
- select statement runnnig fast (for weeks) on v480 server
- suddenly slow for no evident reason (no data/application changes)
- researched...traced/explain-planned
- ...added ORDERED hint...fast again
- wondered/asked about possible causes...no help from DBA...shelved
it/drove on
- server crashed/rebooted two weeks later
- out of curiosity, tried orig statement without ORDERED...fast as day
one.

First, I am not the DBA for this box (I have user level access).
I ask the DBA if he could suggest what could cause a sudden shift in
performance. He had no (useful) answer. There may be a lot of
possibilities here. I realy want to understand what is going on here.
I can provide more detail if necessary. I partly suspect the DBA made
a change and either doesn't know it or won't fess up. I'm hoping there
is some other explaination ... 'learning optimizer'(ha!), etc. Why did
the reboot change/reset things?

Thanks in an advance for any guidance,
Mark