Q: It is missing when I first start it, otherwise it runs smooth.

When you first start the car the engine is cold and the intake manifold rubber seals may leak vacuum slightly causing the rough running and then as the engine warms up the vacuum leak will go away from the pars expansion. Your engine may have other issues but this is one of the common problems with this year of Taurus. I recommend having a mechanic, such as one from YourMechanic, come out for an early morning appointment so they could feel and see the problem happening themselves before giving you a diagnosis. This way they can verify the problem before they do any repairs.