Why is my '05 Mustang GT idling funny?

Two days ago, I had my battery replaced. As soon as the new battery was in, my engine started idling funny. It seems to hover at about 600rpm, then briefly dips down as low as 300, where it's on the brink of stalling. Last night, in fact, it did stall. Other than that, it seems to be running just fine. Any ideas? Could my chip have been "reflashed" when they put the new battery in?