Rover 4.6 v8 oil pressure problem

hi I need some help, my v8 has started to take about 10 seconds to build up oil pressure when cold, it sits at about 25 psi on tick over, when I rev it is goes up a little bit then drops to about 10psi then back to 25 psi soon as I let off the throttle, anyone have any ideas for me? Oil pick up is clear