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