CD667;
Hey, if your engine wants to keep on running after you shut it off, that means you are taking it too easy while you are driving and all that energy is bottled up. Hehehe!

Anyways, the problem could be one of several.
A) your idle is set too high, it should be 800 rpm at idle if it is a standard and 900 if it is an automatic.
B) your compression is too high for the gas you are using.
C) it is running too hot.