I had this happen about 3 times in 1987 before I figured out wtf was going on. I'd pull the oil pump out and find little chunks of off white plastic in the pump locking it up.Didn't know then, but FYI in the 70's ford used plastic timing gears, and when they got old enough, they just came apart, and worked through the screen. Later, I believe they went to aluminum for a while. I know chevy did. Oil pumps don't just lock up...Even an autozone pump. Something gets in there and it's game over. You need to open the pump up and find the problem, or it will continue to eat your time money, and engine.