Strange problem while displaying Flash at IE and FF

Ok, i have worked with Flash almost ever but never saw this prob never…

I´m creating a simple header for a website with Flash which just loads dynamically (php) an image to it. And it has a simple (wave) effect in front of it (the image)…

What´s happening is that while viewing the site after some time the image just dissapear, and my wave starts to have a rough border around it…

It happens both in FF and IE. I guess the time for that happens is not always the same.

Does anyone have some explanation for this?