Play random mc on stage

hello everyone.
here’s the situation:

on the stage i have a few instances of a movieclip.

for the sake of discussion we’ll say 5 mc’s.
lz0 through lz4
what i’m trying to do is use

targetmovie = “lz”+random(4);
targetmovie.gotoAndPlay(2);

but for some odd reason flash does seem to see it.
i can manually say lz4.gotoAndPlay(2);
but i can’t seem to get the “lz”+random number to work.
Ive gotta be missing something really basic…