Disappearing <div> with Opera!

Argh. I've no idea where to start with this one. Take a look at:

http://tranchant.plus.com/notes/cable-wrap

The problem occurs on all pages, but you need a fairly long page to
demonstrate it.

I've recently added dynamically-generated next/prev/up/start links, and
a few "common" links to the header bar. Works fine in Firefox (fixed
header) and IE (non-fixed header).

Opera (7.51) also works fine, until you scroll down the page. Shortly
after the <h1> title goes "under" the header, the top-right links
disappear. It's not like they're gradually obscured - there is a degree
of scrolling at which they disappear.

Any suggestions? CSS is at:

http://tranchant.plus.com/css/screen

The header bar is #header, and the links are in #fn.

--
Mark.