Lightbox background problem

Question

When I put a Lightbox Display Composition at the bottom of the page the background is black at the top of the picture (it appears to be putting the picture in the center of the browser) and once you scroll down to see the caption and the rest of the picture the background is no longer black.  It only happens on Lightboxes toward the bottom of the page.  See below of the examples.
First picture is when you click and open the lightbox.
Then scroll down you see this:
I put a black box because the Caption is white text.  If you scroll up it looks like this:
If you don't scroll all the way down to the bottom of the page it works fine:
As soon as you scroll down past a certain point the Lightbox displays at the bottom and toward the bottom have the same problem with the black not filling the screen.  How do I fix this?
By the way this seems to only be a problem in Chrome.  Works fine in IE and Firefox.
June 19, 2013
This is still happening in Chrome.
Go to http://rheaknives.com/available.html
in Chrome.  Click on the knife in the bottom right corner.  Scorll down on the picture and it works fine.  BUT if you scroll to the bottom of the page and then click the same picture and scroll down on the picture this happens.
Notice the bottom is not blacked out.

Answer 1

Anyone...help?