Sometimes the car hesitates to accelerate

I need some help diagnosing a problem with my car listed above. 
My current problem; The speedometer quits working when driving at speeds above 40 MPH. The RPM fluctuates drastically. The accelerator becomes sensitive. I have to press lightly on the pedal to make sure the speedometer does not go out. 
Sometimes, the car hesitates to accelerate. The pedal is all the way to the floor and the car does not pick up speed. But in neutral or parking, the engine revs.
Past problem(s) months back, the car had trouble starting up. It would sputter and the car would not turn over. After a short period of time, the car would turn over. It did not work well in the heat. Sometimes, the car would stall at a stoplight.
From the forums I have read, mechanics were not successful to duplicate the problems. I have taken it to a Nissan Dealer and 2 mechanics and they were not able to solve these problems. In the past three years,   I have replaced the catalytic converter, decarbonated the throttle body, changed ignition coils and spark plugs, cleaned the fuel injector(s), and replaced the fuel pump. Any guidance would be appreciated.