Ford Taurus Repair Question

My 1999 Ford Taurus SHO car has 89,000 miles.
The check engine light has been on for over a year, but I was told to add additive to the fuel to clean build up. I have not had the light reset, which I believe I have to do. The problem I am having is that when I come to a stop, the car idles fast like it is going to die. It did this for about 2 weeks, but now is actually dying when I do come to a stop. The car will restart right away.