Nice menu pops out of its div

Hi there,

This might be more of a css question than a Nice Menu question. But help would be very very much appreciated! :)

I have a website that looks fine on my machine in all browsers but not fine on a small number of other machines. Basically, machines that have a slightly different system font for Arial.

This is the site: http://springfieldconvent.co.za/

You can recreate this issue on your side by holding CTRL + SCROLL (backwards and forwards to change the font).

After a scroll in either direction, the images below the top menu bar jump left or right due to the menu bar text pushing out the slider images by about a pixel. I guess this is because the nice menu is jumping out of the header and into the area below.

How can I fix this??

Any help would be much appreciated!

Thanks!
Guy