What is the triangular part in a Ford Focus 2001 that keeps it from dying?

I have a ford focus 2001. It will randomly die on me.. and it's almost like the idle gets too low. It's an automatic. SOmeone looked at it and told me I needed a triangular shaped part and it would solve the problem. It's 500 bucks. Does anyone know what it's called?What is the triangular part in a Ford Focus 2001 that keeps it from dying?Flux Capacitor