We use wordpress since 2006 on site hermandadblanca.org, we receive more than 200.000 visits per month.
A few months ago several lectors, and users complain me that from time to time they cant see (cached) or work (not cached) on pages from site because only thing they can see its a blank page.... after some minuts page begins to work again.
This problem happens from wordpress upgrade to 3.0 (but i cant be sure) or maybe from we make the site becomes multisite.
I still searching for a fix but i cant found anything.
We work on a dedicated server with enough memory and cpu, at the moment we have define('WP_MEMORY_LIMIT', '512M');, but we have raised memory several times for testing if it is the cause, and problem still the same... we use wordpress 3.3, wp-super-cache, wp-minify, and many other plugins....
Any help will be very appreciated...