2005 Mercury Grand Marquis hesitates when stopped at red light while pressing the accelerator, what is the cause?

Yes it runs ok till slow down to a stop again.

sounds like map sensor very common to go bad very easy to replace two screws

Thanks I will get that checked out and let you know if that's the problem.

Possible Manifold Absolute Pressure Sensor (MAP Switch). It will jerk at low speeds and run smooth at speeds above 45 +-. Some times it shows no problems codes when checked.