2004 Ford Taurus hesitation when accelerating and stating

Please help, I have a 2004 ford taurus when I start the car it acts like it wants to die, after a few minutes it fine however there is a big hesitation when going into second gear. I'v replaced the throttle positioning sensor, plugs, wires and coil but problem is still there. When replacing a throttle sensor does it need to be calibrated, if so how do I do this. Do you think that this is the problem or is there another sensor I need to look at? Thank you and god bless.