3d box in flash cs4 has dissapearing sides

I made a 3d box using flash cs4 (ive made one using imported images as the sides and another one using vector siding made in flash) Both times, when ive finished the box and rotate it, sides will dissapear then reappear, depending on where the box was in its rotation. For instance, when you look at it dead on, the front side seems opaque, but when i start rotating it, the fron side, at certain angles, becomes transparent and you can actually see INSIDE the box thru the wall, then as it moves it becomes solid again. I am trying to make a presentation for work of a package, rotating 360 degrees on its y axis, but keep getting these odd disapearing sides. does anyone know of this or have any ideas why it is doing this?