Dodge Ram Oil gauge-pressure...Help

I have a 2000 Dodge Ram. My issue is at cold start i noticed when I start it it has a top end tick until the oil pressure gauge comes up. Now while driving it today the pressure dropped to 0. Then I shut it off for a few seconds and the pressure is back up to 50. Then after a few miles the pressure drops to 40 then zero again. Shut it off again for a few seconds the its OK for a few miles then drops to 40 and then zero. So do the Ram trucks with a 5.2 have oil pump issues? Or could it be a oil sending unit issue? Can the oil pan be dropped without pulling the engine? Help