Nissan Xterra 1999-model is sluggish while in motion and stops too.

It moves sluggishly and stop while in motion. But when you crank the engine afterfew minutes, it comes back tlife & still repeat same at intervals as it moves. At first they said it was the distribution head which was changed & it started moving a little bit faster than before. But still stop. So they are suggesting the ignition which is far from being fuel filter also. Kindly advise me on what to do next in order to achieve the much desired success. Thanks & remain Blessed always.