1989 Ford Bronco low oil

Engine problem
1989 Ford Bronco V6 Four Wheel Drive Automatic 145, 00 miles

the low oil pressure light came on so I checked the oil it was ok so iread on the internet low oil pressure switc was known to go bad so I replaced with a mechanical so then only got 20 psi so replaced oil pump now when the motor is cold I start out with 49 psi but as the motor gets warm it cuts down to o at idle unless the rpms are kept up what is my problem worn cam bearings