Strange behaviour

Ok not come across this before and it's driving me nuts. Note that this is a work in progress so many of the links are not yet working.

Start here;

http://steven-carey.co.uk/Newsite/about-us.html

So far so good. Now scroll down the page and click on the link at More ... accounting services. Still ok.

Now use one of the links in the second block of navigation links which in fact are anchor links to the sections on this page.

If you are using Opera, everything will still be as it should be. If you have used Firefox or IE you will notice that the corners from the header are missing and extra ones have appeared below the footer.

What has effectvely happened is the mainframe div that wraps around everything has shiffed down chopping off a piece off the top and revealing a piece at the bottom that it shouldn't.

Can anyone throw any light on this behaviour?

It works now in Safari.

Thanks for the feedback.