Flash photo fade performance

Hi guys,

I'm wondering if anyone can tell me why my Flash photo gallery is experiencing major lag on some photos, and not others.

This is the development version of a website I'm working on:

http://fullframe.com.au/a/ffsite01.html

To test - please go to Gallery > Advertising, and then click the three images at the bottom of page.

You will notice the first and second photos have a jumpy and undesirable fade, but the third, which is the largest of all - has a smooth fade?!

The code I'm using here is:

var tw = new mx.transitions.Tween(whichPhoto, "_alpha", mx.transitions.easing.Regular.easeOut, 0, 100, 1, true);

I've also tried removing "true" from the end to use a number of frames (instead of fading for 1 second, as that script above says, fade for 7 frames for example).  This produces an identical result.

The following website seems to have no problem with even larger photos, and they seem to be using the same tween class:  http://www.brook-pifer.com/

I'm beginning to think it's because that one photo that fades well is larger than the screen size?? I just want a nice, fast fade like on Brook Pifer's site.   Please help - 500 points if you can answer this!