P38 Range Rover Lumpy/Hesitant Engine

Hi everyone just wondering if anyone has any experience or knowledge of the 4.6 P38 engine which is usually smooth and unstressed, developing a lumpy, reluctant nature. Originally the problem, only slight, was apparant only arount the 1500 - 2000 rpm range but quickly has developed over most of the range and typically is on a light or steady throttle. I have run, I believe very low on fuel twice just to gauge the range from the tank and therefore an idea of the MPG. Only had the car 6 weeks it is an N/96, 135000 miles good history and doesnt appear to be short on engine oil or gearbox oil.