2006 Nissan Pathfinder SE off road model

I have a 2006 Nissan Pathfinder SE off road model, The meter that indicates the Oil pump pressure never rise more than halfway at initial start and when the engine is steaming, the meter drops from the half position to almost the lowest position, but when you initiate drive, the meter rises to half position and stops there.
In this condition when driving at about 1.5rpm the engine will develop a strange sound for a short time (about 30 seconds) and stop, but continue again and stop when you accelerate. Though it still drives very fine and smoothly, could there be a problem with the oil pump? kindly advice me please.