1999 Pontiac Grand Am mileage 120000 dying at slow speeds

My 99 Grand Am has begun dying periodically when I slow down for a stop or a turn. It will start right back up, but any time I start to idle down, it will start act like it's going to die. It doesn't do it idling in park or at higher speeds. I've already had the spark plugs and fuel filter replaced. The guys at AutoZone hooked up to it and it gave an error that the fuel mixture is too rich. How do I fix this?

It is a 99 Pontiac Grand Am, 4 cy auto, mileage 120k.

Thanks