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...