1999 Ford Taurus backfiring

I have had this car about 2 months. It runs rough when you first start it up. And when you start moving in it with the engine cold, it backfires up front.I had it checked for codes and it showed up the O2 sensor Bank 1 sensor 2.I had that replaced, as well as a vacuum line with a whole in it.I also had the plugs and wires checked. It clears up on its own after we are on the road a short time. But I know with it running like that for that short time is probably why it gets less than 16mpg Any solutions? I have the 3.0 engine with the U engine code