Loading homepage destroys layout – at least for two users

Hey there,

a friend of mine is running a blog with me. Since we moved from Joomla to WordPress (3.04) he is experiencing serious loading problems when viewing our blog.

Normally, the site has to look like this:

https://bleib-passiv.de

(or in case that you can’t see the right layout neither, here’s a screenshot from my browser view:
https://img821.imageshack.us/i/websitekorrekt.jpg/)

But he often (not always) gets this:

https://img534.imageshack.us/i/ladefehler.png/

He also told me, that the same happens (again: not always) when viewing the page with his mother’s PC.
They both run Windows PCs. My friend uses Windows 7 and the latest firefox version.
I run Mac OS X 10.6.6 and also the latest firefox version.

Now, I can’t really figure out what exactly the problem is. Another blog-reader also told us, that the page is not loading very fast (which I experience, too) but at least he gets it displayed correctly. And so do I.

Any ideas?