onClipEvent = choppy

I’m trying to do simple pan movement of an image, but I’m getting a terrible choppy effect.

Here’s my AS on the movieclip itself:

onClipEvent(enterFrame) {
     xx = _global.xspeed;
     yy = _global.yspeed;

    _root.background.picture._x += xx;
    _root.background.picture._y += yy;

I even tried bumping up my framerate to 50 (it’s already at 30), but it doesn’t seem to do anything. How can I move a MC in AS slowly without it being choppy?