9i optimizer problems

We've been using cost based optimization since Oracle 7. Our upgrade from 7.3.4 to 8.1.7 was fairly painless but the upgrade to 9.2 is proving troublesome. The main problem seems to be that the 9i optimizer is making decisions that can result in order of magnitude response time increases (not to mention temp space usage) in some queries.

The queries that I'm dealing with are quite simple and have no hints. I've already reviewed the "advisor" views and thrown more memory resources at the database. It seems reasonable to me that the 9i optimizer should work at least as well as 8i.

Has anyone encountered similar problems with Oracle 9i?

And yes, the tables are analyzed.

-- 
Tom