Engine will "hiccup" for lack of better words, sometimes very subtle and sometimes very pronounced; does not occur all the time and does not give trouble codes. Car is 2006 2.5 ltr, base Altima with manual 5 spd. Does not matter whether AC is on or off. Mileage is about 140,000. We are the original owner although the car has been moved around the family including oldest grandson. My thought is fuel delivery. I would appreciate your input, thank you.