The SC cars have 2 MAP sensors....one in the supercharger inlet (MAP) and one to ambient air, just behind the supercharger, behind the SC body and the valve cover (called the BARO sensor, for barometric pressure). It also has a TMAP sensor, which is on the intake manifold near the supercharger snout (pulley side). I am still not completely certain if the MAP and BARO sensors are the same part number or not. Some websites list interchangeable part numbers, and some do not. They are cosmetically identical. The Tmap sensor is different and can't be confused with the others.
The plugs to the MAP and BARO sensors are identical, but the wire harness is designed such that you can't really mix them up without seriously stretching wires. Also, if you mix them up, you'll get a code for a pressure mis-match, or something like that.
My car exhibited very similar symptoms to yours, and was throwing a code for the BARO sensor. I have been unable to solve it after 6 months of troubleshooting, and replacing every part in the system.
The good news in all this, is you can just block off the vacuum solenoid, and tune around the problem. That's what I have done. It's not ideal, as it eliminates the PCMs ability to limit boost in a trouble situation. But if you're desperate, as I was, it's an option. My car works great. Until I push it too hard and blow it up.