98 Ram Oil Pressure problem

My oil pressure keeps dropping to zero after a few minutes of driving. I have 105,000 miles on it. I have changed the oil pressure sending unit, plenum gasket, pcv valve, and oil pump. I had alot of sludge in the engine which I cleaned out when I removed the intake manifold. I also took off the valve covers and cleaned any sludge out. I do hear the valves start making noise if it runs with the pressure down. If I turn the engine off and back on, the pressure goes back up for a short while. No smell of gas in oil. Any suggestions???

would probably have better luck posting in the Ram section