My 97 Nissan Altima GXE has started to act strange. At times it is hard to keep it running. When you start it sometimes it runs fine but after 70 -90 miles it wants to stall out and run irregular/ choppy. When I get to a light the rpms will flutter between 600 and 900 and then stall out. It will start up again but I have to work to keep it running. Other times it runs smoothly I have had it into a reputable mechanic who has done all kinds of diagnostic/ cleaned out all the carbon/ checked sensors/ Cleaned Mass airflow monitor. I am not a mechanic and seek advice. Thanks