Most likely is a Moz error. As far as I know, Roger (Moz crawler) automatically adjusts the speed depending on the site performance, I guess watching for the response times, as they crawled sites too fast, they could be either bring down your server or have their IPs blocked as a security measure. It does that automatically. However, sometimes our Websites can become slow and therefore instead of a 200 status code (success load) it gets a 404 as the server isn't responding.
Are you behind a shared server? Were you able to see Roger crawling your site? There are some free services that allows you to make some stress tests to see if your server is responding well, even under pressure.
This shouldn't have anything to do with the fact that the pages are not showing in Google, but, if you are having server issues with regular downtimes, Google may remove pages that they consider gone from your site. Google already said that a couple of 404 errors isn't enough to remove a page from their index, however, a 404 over 3 - 5 days might.
Have you verified your site on GWT? They do show crawling errors, and if they are also getting 404s they should appear there.
Hope that helps!
