Oil pressure issues

My 2003 Dodge Dakota SLT with 4WD has a problem keeping oil pressure. It started slowly with the oil pump not picking up in the cold. It would start up, and just after picking up the oil I would hear a chatter. The oil pressure would hold once it warmed up. I have always used 10W30 pennsoil in the truck, and have changed regularly since it was purchased with 400 miles on it. I currently have 135,000 miles on it. The problem has now gotten worse, and it will not hold oil pressure. The gauge indicates that the pressure is stable until it is put under a load. Then the oil pressure will rise to the high mark then fall completely off. I know that the oil is not being picked up because if I keep the motor running the lifters will begin to tap. I can shut the motor off, and restart it and the oil pressure will return until it is under a load again. At an idle the motor sounds fine, and there is no noticable lifter tap. I have not seen an oil pump do this, but it is as if the oil is not getting back to the pan. It has never burned oil, and still does not show any signs of having done so. My thought is that the return ports have clogged up, but that is my Chevrolet opinion. Please help me understand what this is.

DakotaSLT