Postgresql performance degrading... how to diagnose the root cause

Hi,

I have a postgresql database (8.4) running in production whose
performance is degrading.
There is no single query that underperforms, all queries do.
Another interesting point is that a generic performance test
(https://launchpad.net/tpc-b) gives mediocre peformance when run on the
database, BUT the same test on a newly created database, on the same pg
cluster, on the same tablespace, does perform good.

So the problem seems to be limited to this database, even on newly
created tables...

What should I check to find the culprit of this degrading performance ?

Franck