Lightbox position issiues

Is there a way for the Lightbox to appear always in the same place when the css position is set to fixed?

When a lightbox link is further down the page the lightbox shows up on the bottom of the page and when the page is scrolled to the bottom and a link is pressed it only shows half of the lightbox. It is still fixed but not on its correct original position.

I just want the lightbox to be fixed when you scroll the back content.

Is there a way around this?

here's the site www.pvxg.net

thanks