YOu did not state what engine you have. Here what that looks like on a 3.8... this is just at the intake manifold, where it joins the flexible tubing from the air filter....
www.avigex.ca/xport/maf.jpg
www.avigex.ca/xport/maflocation.jpg
There may be other causes to what you describe, but in my case, that was it, and this is after everyone at GM service and other places were ready to go for fuel pump, fuel filter, computer, .........It cost me just a little patience and 2 drops of alcohol....If built like mine, the active part is just a tiny shiny string in the path of the airflow. It was covered with a coat of clay like material that did not belong there......
THe 3.1 does not have a MAF sensor: it uses a MAP (manifold pressure) sensor.......
EGR valve is also an easy culprit to investigate, very very often the cause of serious engine irregularities.