It usually occurs when the engine is warm, I usually i notice it after the first short Journey so as you said before it could just be the oil before it pressurises.
I think that all i can do is do all that has been suggested and see if the cars better then.
As morty said i do need an induction kit to torque the car up a bit but im still waiting for oldschool to get back to me. Im getting a bit worried now because im going to the RR Remap day and i wanted it on there before hand.