Did you clean or play with the butterfly on the throttle body at all? I cleaned my throttle body once way back when I had 210k on it trying to be nice, the cobalt didn't like that, the cobalt computer can relearn the idle but it might take it a while. when you get to a stop let out the clutch a little with the brakes on until the rpms drop to where you want them, hold that for a second or two and then you can push the clutch back in and it should hold the lower rpms but the next time you stop you will have to do it again. Mine took a few months to learn the lower RPMs. this was in the Cobalt when I did this and I was daily driving it so in a vehicle you drive every now and then might take longer. Others may have a different way to do it but this worked for me.