Why is my Header Shifting Down when I scroll?

Hi, I've been working on my wife's website for a while now. I've noticed an issue that I can't for the life of me figure out. Her website loads just fine, however sometimes when you scroll down the page the site "shifts" and leaves a very large gap at the top. I've been able to replicate this on both IE 7 and Firefox 3 (I haven't tried IE6). I have attached two screen-shots. The only difference between these two shots is I scrolled down the page using the mouse wheel, nothing in the website itself was changed.

One other thing is it doesn't do this all the time. In the case of the screen-shots I reloaded the website and now it doesn't shift at all no matter how much I try to scroll up and down.

Does anyone have an idea where I should start looking to fix this? I've done some web searching but so far haven't come up with anything.

Thanks,

Robert

Website Link:

www.adamicollection.com

You might check this thread:

http://www.zen-cart.com/forum/showthread.php?t=102516

Thanks, I looked at the thread and it looks like it might be an issue with Image Handler. Someone in that thread suggested to disable IH from zooming small images (Configuration-Images-IH Zoom Small Images). I did that and so far have not been able to duplicate the bug in the last couple of minutes.

Thanks again.