I have a 2000 model 4.0 ohv explorer under 90000km. Engine rattles and runs on 5 cylinders for around 5 to 10 seconds on cold starts (more pronounced when parked on an incline). When its warm the engine sounds and perform perfectly. Oil pressure is good and consistent. I recently changed the oil (it was very dirty) but fresh oil made no discernable difference. Any ideas/suggestions most welcome