Perfomance decreasing

Hello.

       I have a question about perfomance.
       I'm running PostgreSQL 7.1.2 at FreeBSD 4.3.

       For  the first 1-2 days of running perfomance is excellent. But
       after  that,  speed  began  to  decrease.  And  after a week of
       operation, perfomance  falls  8-10  times, than at first day of
       using.

       I'm  doing  vacuum  periodically  (once a hour), but perfomance
       still falls down.

       After that I dump database as text file, make dropdb & createdb
       and  after  that,  restore  database from dump -> Perfomance is
       excellent again (for 1-2 days).

       Why this situation occures? May be I must use "VACUUM ANALYSE"
       instead of VACUUM?
       
Best regards,
 Alexander                          mailto:sas@mplik.ru