Idle up and down

1974 Duster 360 4-speed. Trying to get a few opinions on this problem. Car runs pretty good , but when I come to a stop after its warm the idle will stay high and then drop down to a slow idle. The throttle is returning properly-return spring working properly. Eddy 600cfm. Carb fairly new but same issue with old carb. Is this a symptom of something common. Also just did a tune up-plugs wires cap rotor ect.