Performance suggestions?

Hi all,

I'm after suggestions on how to keep the initial performance
of a system I've put together for longer than 2 minutes.

I have a small table about 20 rows, a constant, that is receiving
about 160 updates per second.
The table is used to share gathered data to other process asynchronously.
After 5 min it is 12 updates per second.
Performance returns after a vacuum analyse.

I'm using 7.4.5.
This is the table structure
Table "public.lastscan"
Column | Type | Modifiers
-----------+-----------------------------+-----------
pointnum | integer | not null
parameter | character varying(8) | not null
value | double precision | not null
dt | timestamp without time zone | not null
Indexes:
"lsindex" btree (pointnum, parameter)

Or should I just stick to saving the data, inserts seem to go on and on, and
use a different IPC method.

Ta
Allan