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(at)mplik(dot)ru