Slow INSERT

I am experiencing rather slow INSERTs on loaded server. The table I am
inserting to is:

CREATE TABLE pagestats
(
page_id int4 NOT NULL,
viewed timestamptz DEFAULT now(),
session int4 NOT NULL
) WITH OIDS;

The table is populated with 700k rows. It is VACUUM ANALYZED every
night, though it is only INSERTED to and SELECTED from, no UPDATES or
DELETES. There are no indices, triggers or constraints attached to it.
There are about 5 inserts pre second (sometimes more, but 10/s max).

The INSERT is:
INSERT INTO pagestats (page_id,session) VALUES (5701,1147421823)

Sometimes, it takes as long as 1300ms! Other queries are quite swift,
even compplex SELECTS and most of the INSERTS run fast. But occasionally
(every 50th or 100th INSERT) it takes forever (and stalls the webpage
from loading). The only special thing about this table is, it does not
have a PRIMARY KEY, but I should think that this constraint would only
slow it down even more.

Any ideas what can be wrong?

--
Michal Taborsky
http://www.taborsky.cz