That's very interesting John 'cos I think that may bear out my theory that lack of oil, or rather, lots of it in the wrong place, may be the cause rather than temperature. The interesting thing was that the pressure would drop after a moderate period of high sustained revs, e.g. Lavant straight at Goodwood, and return to normal very quickly after slowing down. Having since fitted an oil temp gauge it is obvious that the oil temperature takes far longer to cool down than would account for the rapid recovery in pressure. Paul, I would try John's tank method first, or fit an oil temp gauge to be sure that temp is the real problem, before lashing out on a cooler.
I know there are lots of people out there who have never had this problem, even though they drive extremely hard. I wonder what the variability of the engines is to cause this.