Trouble with Movie Clips whos depth i am changing

hi all

i am having trouble with movie clips whose depth i am changing with swapDepths().

They act a bit irradically and do not always stay invisible even if i set _visible property false.
Also they tend to come back on stage even after i remove them using removeMovieClip(Mc), specially if i go back on the timeline (though this is not a hard and fast rule).

Can anybody guide me.