i am going through the exact same problems with my gf 99 amigo 3.2. I called st. charles and asked him about it and I suggested a fuel pressure regulator. He said if it doesnt leak, then it most likely isnt it. What he did say is that it is most likely the Intake manifold gasket or the MAF. I have sprayed carb cleaner like crazy all around the gasket on the manifold with no rpm change. So im leaning towards a MAF sensor (would this cause bad gas mileage?) but, the problem has gone away so now i dont know. What i do know is that she is getting 13 MPG!!! i have cleaned it like crazy and cant get it to be right.

so if was you, id check the IMG by spraying carb cleaner (o2 sensor friendly) and see if the rpms change.

kevan