89 grand prix 2.8 loss of oil pressure

I have a 89 pontiac grand prix with a 2.8. I am having trouble troubleshooting this easy problem. When I first start it after sitting all day the oil pressure is around 60psi. When it runs and gets to tempature, the oil pressure goes down to nothing and the car stalls. You have to let it sit and cool down before it will start again.It always starts back up after it cools.
O.K. here is the deal. I have 218,000 miles on this old girl and I'm not ready to let it DIE. In the last few months I have changed the plugs,wires,radiator,water pump,fuel pump,vaccum lines under the plenum ,hamonic balancer,serpentine belt,intake gasket,thermostat,hoses,MAP sensor,O 2 sensor, and I keep the oil changed.
The car dont make a noise and runs great except this new problem and I think It might be the oil pump because of the miles.Will the oil sending unit make the car gauge lose pressure and shut the car down? What about the crank and cam sensors?Any help here is a timesaver on me,thanks
My goal is to get at least 250,00 miles out of her before I let her go! HAHA!!

Welcome to AF!!

The problem may be with the oil pressure sending unit that is located on top of the housing where the oil filter sits. If you have 60 PSI at start-up then something else is going on. The sending unit is interlocked with the fuel pump circuit so that if the pressure drops too low the fuel pump will shut down and shut down the engine. I would change the unit and then you may want to try a 10W40 oil instead of a 5W30 so the oil will be a little thicker when hot if the problem persists after changing the sending unit.

I changed the oil sending unit and I still lose oil pressure but the car no longer stalls just idles low. I will put more miles and get it hot to see if the problem gets worse. Any other advise on what to do next?

I would look to the oil pump. But depending on how the fuel pump is tied into the oil pressure sending unit the problem could have to do with the pressure gauge (if it is mechanical). What I am saying is that if the signal for pressure goes from the gauge to the fuel pump rather than from the sending unit itself the gauge could have faulty readings when it gets hot thus sending an incorrect signal to the fuel pump. But I would to a mechanical problem first.