I also have this problem. I have a 2008 Impala LTZ with near 88,000 miles. When taking off from a complete stop and sometimes just when depressing the gas pedal fairly heavily while driving ( say when turning at an intersection at a slow speed) the car is not reactive to the acceleration for a quick second, and then it will jolt forward. Also, the shudder that others have experienced when accelerating also happens to me with my car. I, too, took it to the dealer since I just purchased the car outright and was told that there are no codes and the mechanics cannot reproduce the issue and therefore , basically, there are no issues as far as they are concerned. I am thinking about selling this car while it's still got under 100k simply because of this issue. Has anyone seen this car maintain operation for any length of time despite this issue? I mean, will I get another several thousand miles on it despite this issue or should I sell it now? No one seems to have the answer to the issue..