Oil pressure shoots to the high max and then drops to the lowest it can

Okay, I thought I was a descent mechanic but I am stumped. My father's truck has been raising to the highest the gauge shows and then it drops to the lowest setting on the gauge. I thought the gauge was wrong maybe but it only takes a minute and then the temperature raises. Which means since it has coolant it is not getting oil. I have replaced the oil pressure switch and even did a oil change with no success. I bought the oil pump and realized I do not know if I can put it in, in my driveway and simple hand tools. Any thoughts would be appreciated.