Vcam messes up!

Vcam is working pretty good, until i add y values to it.
here is the code where it works fine.
**
onClipEvent (enterFrame) {
_x += (_root.hero._x-_x)/4; }**

but when i add the y values as such:
**
onClipEvent (enterFrame) {
_x += (_root.hero._x-_x)/4;
_y += (_root.hero._y-_y)/4; }**

what happens is MCs just fall off the screen, even though the hittests for the gravity are working fine.