Gaz,
I have experienced something like it in the past, when a cheap and nasty oil gets hot, like after a fifty mile journey and then you demand the engine to do some serious work like racing a jag! What can happen is your oil is now too thin and will not feed all the way to the Hydraulic tappets. I was a bit short of money a few years ago and put in some cheap and nasty oil from a German Supermarket. I then couldn't understand why the buzzer under the dash and the pressure light was comming on. I changed loads, oil pressure switches etc. At the end of the day I simply changed the oil for a good quality mineral oil (GTX) and the problem ceased immediately. The engine is going very strong to this day. Try a simple oil change first.
Regards Mick
