[scicala]

Did you try adjusting the speed screw ? You only mentioned the mixture screws which aren't really for changing the speed, but for trimming the mixture for smoothness.
Besides that, it sounds like the throttle lever is not returning to the same spot every time you return to idle.

Check to make sure the return spring is doing it's job, or for a loose peen where the throttle lever attaches to the throttle shaft.

Also could be loose throttle plates as George/Maine mentioned, although would be unusual.

Sal