RDBMS Version: Oracle9i Enterprise Edition Release 9.2.0.6.0 - Production
Operating System and Version: Windows 2003

Performance degrade after migration from oracle8i

I have shifted database from oracle 8i to 9i thru export and import.
I also ran gather stats thru command

execute dbms_stats.gather_schema_stats(ownname => 'username',estimate_percent=> 33,cascade => TRUE,degree => 4);

The database ran two days perfectly. On third evening database performance degraded drastically. I check buffer hit and cached hit, which were above 99%. Then I ran gather stats again and performance restored. Now fourth evening database performance down again so I ran gather stats and database start performing.

Any idea where things are wrong....Regards