I've been told by many people that the best way to keep a modern computerized car consistent is to keep it running the whole time. We had a guy at Norwalk racing a new 300C hemi a few years ago running within .05 all day. He said it was .3-.4 slower than when he kept it cool but VERY consistent when he let it run.
I haven't tried that with my Cobalt. I'm too cheap to keep it running and waste gas. It'll run pretty tight for a few rounds, the go + or - a tenth or more for no good reason.