Ubuntu 12.04 / Magento 404 error js and css folder

Question

I'm having a strange issue with my Magento webshop that after a period of time, the webpage cannot find the css and JS files anymore. They all give a 404 error. This results in prices displayed in USD instead of EUR and the admin page not reachable. This issue occurs once a week/ a month. I see that in the link, the "js" folder is missing. So instead of www.domain.com/js/mage/adminhtml/form.js, the webpage tries to reach www.domain.com/mage/... resulting in a 404 error. The issue is resolved when I restart the apache service on my server. Not when I clear the cache.

Any idea why this happens? It started a few months ago when I started using virtualhostfiles to host different websited on one server.

This is my file:

<VirtualHost *:80>
     Documentroot /var/www/shop
     ServerName shop.test.be
     <Directory /var/www/shop>
        Options FollowSymlinks
        AllowOverride All
        Order allow,deny
        Allow from all
     </Directory>
  ErrorLog /var/www/shop/error.log
  CustomLog /var/www/shop/access.log combined
</VirtualHost>

Sincerely

Jossk