depending on where you're calling your action from, you may be using unnecessary nesting to control your movie...
for example, if you're on the root timline, not inside any movieclip, and you're assigning an action to a frame to control your "MAIN_MC" movie clip, then I BELIEVE you would do this:
MovieClip(root).MAIN_MC.gotoAndStop("portfolio");
I've actually got to tackle this issue when I get home as well, so if that doesn't work, I'll have a better solution when I get home when I have flash in front of me.