404 Error with Post that's there and live

So, getting a bunch of 404 Errors being reported from Google Webmasters, so decided to check them out… and this really has us baffled.

I check the URL listed in Webmasters, and when going to the post, it gives the 404 error (they use to show up – though not sure when this started happening).

Log in to the Admin Panel, then find the post, open it, click “Update”, and now the post appears (no 404 error)…

1.) Is there some way to update these post faster than having to manually do it for every single one?

2.) Why would this have happened? It seems very random on which posts are doing this (post length, images/no images, etc), so we would like to avoid this in the future.