WordPress pages randomly serve as RSS/XML content

For months we have been struggling with a weird issue on 1 of our WordPress Network installations. We have 4 WP Network installations on 1 VPS and only one of the three installations generates this error.

The problem is that pages on all sites that run on that WordPress Network installation, randomly(?) seem to try and parse the page as being RSS content. See this screenshot: http://i.imgur.com/XvZlh.jpg

This only happens once per session (if it does at all) and after refreshing the page, all is good. This makes it extremely hard to fix, because debugging is almost impossible.

Could this be a WordPress issue, or could this be an issue in the Apache config?

Some domains we are encountering this on:

– http://www.skynetsg.co.za/
– http://www.workplacestrategies.co.za/
– http://www.beachbreaks.co.za/

I tried upgrading from WP 3.2.1 to WP 3.3, but that didn’t make a diffrence.

I hope there is someone that could help me solve this issue!