CKEditor for ASP.NET occasionally doesn't load

I'm using CKEditor in my ASP.NET 3.5 (VB) website and I'm finding that roughly once in every twenty page refreshes the editor doesn't fully load and leaves a blank space in its absence. When I look at what's happening in Firebug I can see that the textarea is hidden and the span that holds the CKEditor has a class of .cke_skin_kama which has its visibility set to hidden. When I expand /ckeditor/skins/kama/editor.css?t=C6HH5UF to view the CSS I see a 500 internal server error.

I'm using Firefox 19.0.2, I have the latest version of CKEditor and I followed the installation instructions on the documentation page. I'm using two editors on my page but I've reduced this to one and it makes no difference. I've tried removed the .htaccess file as instructed in a post I found but this didn't help either. Most of the time CKEditor works perfectly but every so often it doesn't load and I would really like to fix this issue. Any help would be appreciated.