I'll only claim to know some mere basics about superchargers but i claim to know nothing about GM superchargers, especially for the cobalt.

My boyfriend is interested in a used 2006 Cobalt supercharged, it's got 32,000+ miles on it and is reasonably priced. When we went to test drive it we pulled over in a parking lot to switch drivers to see if i could feel anything different with the handling of the car. When we were parked the car was while idling was shooting up to about 2k rpms and then it would drop down to just above 0 and the car would begin shuddering. Not wanting to stall out and possibly be stranded in the middle of St. Pete we drove right back to the dealership and told them about the problem.

We just now found out, after the dealer took it to a mechanic to fix the problem, that the previous owner had upgraded to the stage 2 supercharger (lightweight pulleys, flashed ECU, ect). The car is immaculate and very well taken care of but the shuddering when the car idles doesn't seem normal.

Neither of us having ever owned a supercharged car before and are pretty much noobs to whether the dealership is trying to pull a fast one on us by saying that the car is supposed to do that with all the upgrades on it or if they are actually telling us the truth.

Here's a link to the car:
View Inventory

Thoughts? Help?
Thanks in advance guys.