How to go around this?

Hi all, im having this weird issue with my movieclip and i was thinking that maybe one of you wise ones have solved this:

i made a movie clip scroller and whenever i drag a button, it scrolls my movieclip. simple. The problem is that flash is not fast enough. I have this kind of statement in the onEnterFrame:

if(mc._y <= 0) {
return
} else mc._y += speed;

speed is something like 3,5 etc…

PROBLEM: the mc sometimes goes past the 0 point. sometimes when im lucky, the onEnterFrame happens just in time and it stops to 0.

I tried this using onEnterFrame and as a function with setInterval but it is always the same.

Is there a way to around this? i could make mc._y to 0 before the return statement but that looks very dumb…