Strange performance degradation

Hi all,

I'm experiencing a strange behavior with my postgresql 8.3:
performance is degrading after 3/4 days of running time but if I
just restart it performance returns back to it's normal value..
In normal conditions the postgres process uses about 3% of cpu time
but when is in "degraded" conditions it can use up to 25% of cpu time.
The load of my server is composed of many INSERTs on a table, and
many UPDATEs and SELECT on another table, no DELETEs.
I tried to run vacuum by the pg_maintenance script (Debian Lenny)
but it doesn't help. (I have autovacuum off).

So, my main question is.. how can just a plain simple restart of postgres
restore the original performance (3% cpu time)?
I can post my postgresql.conf if needed.
Thank you for your help,

--
Lorenzo