Movieclip is not invisible when it should


I have a movieclip on stage (first and only frame in the _root timeline), that is duplicated to create several movieclips. I have a line of code like this mc._visible=false; to hide the original mc. But sometimes when I play the movie, the original mc appears visible, for much less than a second yes, but it shouldn’t anyways.

Does anyone know a solution for this?
Thanks a lot