1998 Dodge Truck oil pressure

I have a 98 Dodge 1500, It loses oil pressure on a regular basis while driving. It will come back on it's own sometimes. I have changed the oil pump 5k miles ago, I cleaned the oil pick up tube and it still seems to loose pressure occasionally. I say it has lost pressure because, after a test run to see if it was the sending unit or actual pressure loss, the lifters did tap.
What is the possible cause of this and what is the flow pattern in this motor with relevance to pump, sending unit, block, heads, and filter?