This means that your website is hitting one or more of the hosting account's resource limits. You can check on this via the 'Resource Usage' icon on your control panel.
We use the 'CloudLinux' system to ensure that no single hosting account can monopolise server resources to the detriment of any other.
If your site is consistently hitting its resource limits, you may wish to look into improving the efficiency of any software you're running on your account, or consider an upgrade to an account type with greater resources allocated to it (e.g. our VPS Hosting Plans).