I have a 2009 GMC Yukon all stock. Under no brake de-celleration the truck shudders badly almost like slipping a gear or doesn't know what gear to select. The problem also happens when on cruise control when de-cellerating and when cruise is set to 30-40 mph. I am under waurenty but 2 trips to the dealership have been unsucessfull. They can't replicate the problem and the first time the said it was a normal test the car runs. Please help