1996 Dodge Truck Oil pressure. Or lack there of !

Engine Mechanical problem
1996 Dodge Truck V8 Two Wheel Drive Automatic

I have a 1996 dodge truck with the 318 magnum. The truck was bought by the original owner in 96 and was driven 10,000 miles before it lost oil pressure and was taken to dodge for service. After the repairs where complete the truck was driven another 30,000 miles, and done the same thing. Once again the truck was taken to dodge and fixed. Although the truck was fixed, and sent back, within 20,000 miles the truck did the same thing, understandably the owner gave up and parked it next to the house. Now I hve the truck and am trying to find out what would cause such a thing. The truck will idel about 30 sec. To 1 min. Before loss of oil pressure happens, and when it does in does not knock on the bottem end, just a tap on the top.