I love this website, because it gives such a great description:
GM Cutlass Ciera/Cruiser/Supreme ? OBD Trouble Codes
34 Manifold absolute pressure (MAP) sensor -voltage low, high vacuum indicated Wiring open circuit/short circuit to ground, MAP sensor, ECM
There is one of two possibilities, either there actually is more vacuum then what there should be, or the vacuum reading is incorrect.
I believe the next step should be to scan live data, and throw a mechanical vacuum gauge on the car at the same time when it is acting up. Either the data readings will be consistent with the mechanical gauge, and doing something funky in unison, or the gauges will not be in agreement with what is going on, in which case you would track down a bad sensor, electrical issue, ECM issue, etc.