Using Mouse coordinates to control Swf alpha

I have a main page which call a couple of swfs into blank MCs. I’d like the swfs to fade when the mouse enters a certain area of the main swf.

I have tried a simple conditional statement using the ymouse property to control the alpha of the MCs holding the swfs. They fade once and never reappear.

Do I need to instruct flash to check the mouse position after every frame and if so what language should I use to do that?

Thanks for any help.