something look very strange in IE6 when support fixed position

Question

Hi everyone! To support fixed position in this website:

I have to set html overflow-y = hidden and change overflow-y of body = auto. Yeah, in normal case it works well. But when I resize window, turn on developer toolbar, minimize, maximize window,..... many times, I got something like this(see image: http://ca7.upanh.com/14.244.18470726.G0B0/image.jpg ):

I had searched a lot, but I don't know exactly what kind of reason make this website look like that. Any help ! Thanks.