Oil pressure

Ive changed the oil pump and oil and filter and when it is first started oil pressure gauge reads 40 to 45 psi and after it warms up the oil pressure drops to 0 and check gauges light comes on for about thirty seconds then goes off and then rises to about 30 to 35 psi. When the oil pressure drops the lifters start tapping. Oil sending unit? 1996 jeep grand cherokee limited with the 5.2 v8 318