Module and functions goes missing intermittently Looking for ideas as to why a custom module might "go missing" intermittently.

It has been put into production, serves thousands of page views and works well most of the time.  Every now and then it just "seems" to go missing.

Symptoms are:


Fieldable panels pane plugins (declared by the same module) can't
make callbacks to functions within the module, getting "undefined function" errors.
Template files which call functions from the module get the same errors above.
The module declares a hook_menu URL which starts returning 404s.
Form callbacks functions in the module get "not found or invalid
function name in drupal_retrieve_form()"


All of this is intermittent, runs fine for a week, or maybe a couple of hours, then it starts happening.  I clear the caches, sometimes once or twice and it comes back to normal.  There are no errors in the logs that I can see, nothing in watchdog that stands out.

How is it possible for a module and its functions to randomly go missing?
I may have tracked this down to memcache, though it's such a strange error I can't be sure.  Sometimes a group of nodes we put into cache was over the memcache 1mb default limit.

So instead of caching the nodes, I ran them through the theme layer first and reduced the required caching space by about 80%.

This didn't exhibit on my local host as I don't have memcache installed, rather relying on Drupals database caching.  Running some queries like this helped identify the potential problem:

SELECT cid, LENGTH(data) AS Cache_Size FROM cache ORDER BY Cache_Size;