Slow Inserts on large tables

I have a problem where by an insert on a "large" table will sometimes
take longer than usual.

Usually the inserts are quick then from time to time they will take a
long time sometimes as much as 10seconds or longer. (But usually under
500ms which is when I start logging them)

The queries are slow drip fed so bulk loading really is not an option,
Its logging data. Used in analysis and for historical purposes mostly.

I think the problem might have something to do with checkpoints, I'm
relatively sure its not when the table expands as I've run a vacuum
verbose straight away after a longer insert and not found loads of
space in the fsm.

I'm using 8.3.1 (I thought I'd upgraded to 8.3.3 but it does not look
like the upgrade worked) I'm more than happy to upgrade just have to
find the down time (even a few seconds can be difficult)

Any help would be appreciated.

Regards

Peter Childs