Sometimes it starts right away and sometimes it does not start

My vehicle is an SE five speed stick shift. When I try starting my car it starts right away and sometimes it does not start. After quite a few tries it starts. After it starts the car runs like new no noise. When the engine is running and the car is stopped you cannot even hear the engine running. There is no sound or noise in the engine while it is running. The is stable while is running. The starter has been replaced twice. It has new spark plugs, new camshaft sensor. New battery. Oil change is done every 3,000 miles synthetic oil. What is the problem?