choaking and stalling while idling

I just bought a 91 cavalier z24 a week ago and it is now running horribly. It turns over fine but then begins to spudder and seems to be choaking itself. When I drive and keep steady on the gas it will rev and stop making the RPMs go up and down constantly. It almost seems like it is a vacume problem. I ran the codes on it and it came back as a map sensor. I replaced the map sensor (the one with the vacume hose and three prong electrical attatchment on it), but still the same problem. Has anyone ever encountered anything like this before. Please HELP!