Pages occasionally being cached as 404

I run a website with 200 static pages for anonymous users using Boost Module. On average, 1-5 of those pages become cached as a 404 error page each day -- even though the URL is correct. I am not sure what is provoking this problem and how to prevent it, I've done several searches and it doesn't seem to be a common problem. I've added the 404 page on the Boost Modules configuration in order to specifically not cache it, but it is still a recurring problem.