Prototype and Position of the Popups!

Greetings

I have a major problem with my popups. I am using the prototype library
and almost all my popups are opened
ALWAYS at the top and the middle of the screen of every page.

The problem is that I have some pages where you need to scroll down in
order to open the popup but it always opens
at the top which is realyl confusing for the user.

I do not get any javascript error so I cannot figure out what the
problem is. Any suggestion ?

Thank you !