Basic tween is glitchy?

I have a simple movieclip on stage which has a circle drawn in it.

Just tweening it from one side of the stage to the other causes a jumpy/glitch effect. I’m not using any easing- if I do use it, the glitch is still there.

But if I do this: myMc.x-=1 on Event.ENTER_FRAME, the movement is perfectly smooth (but far too slow for what I need, even on 40fps, 60fps upwards).

I used to use Flash 8 and have no problems at all (from what I remember).

Any help/suggestions would be appreciated. :}