Then check if you have stats collected in a comparable fashion to the way in which they were collected in 8i. The upgrade *will* have created its standard gather_stats jobs (a gotcha that caught me on my first 10g upgrade as opposed to new install).
I wonder if perhaps you have both object and system stats gathered, and gathered at a time when there was batch activity in place rather than during a typical OLTP day.
-- Niall Litchfield Oracle DBA http://www.orawin.info