90 cavalier 3.1 Problem

I have a friend that has a 90 Cavalier Z24 with a 3.1 engine. He has a couple of problems with the car.One is that he can be driving it and suddenly he loses power and the motor starts to labour. The second is that he can pull up to a stop sign,be sitting there and suddenly the motor will rev up by itself. So far he has replaced gas cap,fuel filter,O2 sensor (twice),throttle position sensor,and tried another used computer in it.Does anyone have any other suggestions? I have not taken the car out for a spin myself but those are the way he describes the problem's.

Have him check for Vacuum Leaks around throttle body also ,check IAC Valve and clean throttle body and IAC port . have him check catalytic converter to see if it is plugged .Have it scanned for any codes before replacing any more parts

Thanks for your reply i know he has taken the cat converter off but not sure about the rest. Will get back to you.Thanks again.