V6 Mustang, Idle Problems, Stalls.

My Mustang has had an idle problem for a while now, I had an EGR code, replaced Egr valve, sensor, and cleaned all lines, and code went away. But problems are still here! The car trys to die out, when stoping, RPMs go down to around 500, then makes a strange sound, like a gulping/sucking, from the air filter intake. Has anyone had this problem? Any help at all is greatly appreciated! Thanks