very slow after a while...

Hello,
I have a machine that uses pgsql version 8.0.1 I don't think the version
is relevant because I had 7.4.1 before and I had the same problem. I have
a PHP script that runs regularily and does this:
select a bunch of lines from a mssql database
insert into postgres the values taken
if insert fails with duplicate id (I have a unique index there) then
perform an update with the values inside.

Pretty straightforward.
The thing is the after I updated to 8.0.1 and also (separate ocasion)
after I recreated the database one day, the script runs instantly with
thousands and hundreds of lines inserted and updated per second. However,
after a while the whole process slows down significantly, eraching the
point of one insert or update per second or per four!!! seconds.

I have tried vacuuming full, reindexing, deleting the table and recreating
it, I tried changing values in postgres conf and in the linux kernel.
Nothing works except re initialising the whole database directory.

PLEASE HELP!

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