My Malibu has had an idle issue where when i stop in drive the engine will seems likes its misfiring and the engine speed will go up and down about 150-200 RPM. This only happens when the engine is cold. Just today i experienced the same thing but at around 30mph and it kept doing it as i came to a stop. I also feel as if sometimes the engine will have reduced power. I drive the same way every day for work and with cruise control set only some days will it downshift on that hill and sometime it wont. Usually around 50mph every time.