Drupal Cloud Cache Boost
Context
The Boost cache writes out html (and caches it) into the docroot, so that on the next hit it can be served more quickly (avoiding multiple database hits). Our configuration caches data and serves catched data over http connections only - https connections avoid Boost altogether.
Some variables, such as from the http host header, are interpolated into the cached document (eg to expand relative links). We've previously had issues with a visitor offering malformed header variables (that nevertheless got to the right SNI host according to apache) that caused incorrect boost caching; we've worked around the known one, but it's possible that other bugs exist in this class of bug.
Solution
To identify an intermittent problem that's involved in the boost caching, my suggestion is to save out the page source, and look for the footer added by boost. It will contain something like:
<!-- Page cached by Boost @ 2015-05-04 13:44:03, expires @ 2016-05-02 13:44:03, lifetime 12 months 4 days -->