This is a legitimate safety concern that merits your (and GMs) immediate attention. Owners of Ford vehicles have reported similar issues. Our firm investigated dozens of these cases and the problem was traced back to the vehicle's throttle body. You can see a news story about that issue here: http://www.wsbtv.com/news/news/local/ford-replace-troublesome-engine-part/nd449/ Your vehicle may have the same problem, but it is impossible to tell without an inspection. Also, GM has...