Nissan Sentra Dies While Driving

hi.i have a nissan sentra b13 with a b14 engine…the car has started cutting off on me at unpredictable times , sometimes when im driving or even when im idling in traffic…. it happens out of the blue.  it starts back after easily i may have to press some gas but not always .. it will cut off today and not again for  a couple weeks may give a brief bad idle before it does but thats it. i have checked the pump, gas filter,air filter, throttle body, gas regulator and injecters… all are goood even my mechanic is confused  plea

se HELP!!!!!!

i also failed to mention that the last time it happened the car was idling in the yard and the bad idling started suddenly but it didnt cut off and i noticed puffs of black smoke coming out the exhaust [the car doesnt normally smoke so i found that strange] ….any other guesses?