High Idling.

2001 Ford Mustang V6@ 79,600miles-My car sometimes sputters when I start it up for the first time during the day.I don't know if this problem is also related with my car idling to high after awhile of driving it. Every time I come out of a shift and stay in neutral it does this, at least until I shift into another gear or I come to a complete stop. Ive had this problem for about 2-3 months now and it has just recently gotten little worse(revs alil higher). I wanted to fix the problem myself so I went to Auto Zone. They told me my fuel injection system might need cleaning, so I bought the cleaner and it did nothing. I always use 93 grade fuel. They are supposed have cleaner in them right? Im sure this is affecting my MPG. Im a college student with low cash flow and would rather fix the problem myself. Do you have any idea what this is? Need help please!