Are your logs generating any sort of error when this behavior is occurring? It sounds like something is stopping the script from being executed during that time period.
Do the users in questions have any sort of formatting to their names that is out of the ordinary? I know that the PHP Suhosin security module will up and refuse to load at all when content on a page matches at all with a keyword (and the default keywords are quite common words); but any actions will be logged in your webserver logs.
