Movieclip within button begins it's timeline from start during mouse over

My animation contains a button which ‘hovers’ or ‘lightly floats’ on the stage.
The object and motion tween creating the ‘hover’ is in a movieclip which I then applied to a button symbol. When I test my swf and scroll over the button, the motion tween within the button skips back to the beginning. Is there a way to get a smoother transition?

PS. I put the button into a movieclip with the a motion tween and it doesn’t seem to work properly.

Any ideas?