i had this problem one time, wheni encountered it i checked the oil,it was severely low, i added oil and it idles at 40psi and at 500RPM...

other opinions

popular beleif is its cam bearings are worn but not worn out...

a kink in the tubing(its happened to me before)

the plastic connector for the oil pump shaft is worn so badly its started to slip, that can come from revving the shit out of it