Need some CSS help.

[ChrisVCB]

Hi guys and girls.
I've been putting together a site for a friend, basing it off the rather excellent picaflor-azul Ashley_Pink template. It's all fine and dandy, but the site is doing something rather annoying that I know is related to the CSS - however, I don't have the knowledge or expertise to fix it myself.

The problem is rather hard to describe but here goes; when you load the page, the header is a certain height. When you refresh the page, or click on a link in the header, the header changes height.

I've attached a screenshot of the problem, but the easiest way is to see for yourselves:
http://www.the-bike-shed.com/

You should notice that, as you browse the site, the 'wheels' of the logo, as contained in the header_bk.jpg image, are intially covered by the Navbar, but if you refresh, or click a link in the header area, the positioning changes to it's intended positioning. This seems to be because the area between the logo and the top of the header area changes.

Like I said, beyond changing colours and values in the existing CSS, i've very little knowledge in this area, and have no idea how to fix this. Any help would be appreciated.