I have a 1990 cavalier z24 when you first start the car the rpms are around 1000 rpm then when you drive for a while when you idle at the light it runs around 500rpm and the motor runs ruff?

My son has an 1989 z24 that was doing the same thing. After changing all the sensors and plugs, plugwires,fuel filter,coils,throttle position sensor i deceided to attempt to change the fuel pressure regulator while releiveing the pressure off the fuel rail i noticed fuel had been leaking around the valve so i took a valve core stem out of an old tire and replaced it started the car up and it seems to be fine now. Try it.