Okay, I have boiled this code down as far as I can and don't get why I am getting a compiler error.

First, the control group.
Empty CS4 AS2 .fla file with the one movieclip in the library "UI". Just a circle for this boiled down test. Works fine, elastic easing.

Actionscript Code:
import mx.transitions.Tween;
import mx.transitions.easing.*; 
stop();

var UI:MovieClip = _root.attachMovie("UI","UI",_root.getNextHighestDepth());
UI._y = 10; UI._x = 50;
UITween = new Tween(UI, "_y", Elastic.easeOut, 10, 200, 1, true);

But I don't want elastic easing for my actual project this is being used on. Changing the ease type causes compiler error:
"The name of this class, 'mx.transitions.easing.Strong', conflicts with the name of another class that was loaded, 'mx.transitions.easing.Strong'."
At location: "Strong.as, Line 8"

The only thing that has changed is replacing "Elastic" with "Strong"

Actionscript Code:
import mx.transitions.Tween;
import mx.transitions.easing.*; 
stop();

var UI:MovieClip = _root.attachMovie("UI","UI",_root.getNextHighestDepth());
UI._y = 10; UI._x = 50;
UITween = new Tween(UI, "_y", Strong.easeOut, 10, 200, 1, true);

I am guessing this is something really stupid I am missing, I need a new set of eyes.

The circle still tweens by the way, it just tweens with no easing as if I had selected None.easeNone

I have also tried using the full path "mx.transitions.easing.Strong.easeOut" with the same result. The last time I had this problem it was me having a bogus .as file in the same directory, but I have tried compiling this in a directory all by itself and same error. I don't get it, such a tiny bit of AS2... Maybe I just need more sleep, heh.