Check to see how freely your cable and linkage is moving.
Is this a monojet (post 70/71?) equipped with an idle-stop solenoid, because if so, it may be malfunctioning, and I would recommend replacing the entire carb with a pre 70' monojet, to eliminate this pesky bugger.
If your carb is not equipped with such devices, check your idle adjustment screw.
my two cents
matt
aka 250Nova