I've just spent the past 6 hours finding out why a live DNN 4.3.5 site went down completely this weekend after working fine for several months with the only changes being the addition of content to existing Text/Html modules and many new events to the Events module. When a user would hit the home page, they would receive either no reply, a page cannot be displayed error, or timeout error. A look at the EventsLog in SQL management studio revealed a multitude of System.OutOfMemory and SQL deadlock exceptions as well as frequent Application Startup events. All files and SQL connectivity seemed to be intact. Shared hosting setup looked fine. Hitting a html page or even the KeepAlive.aspx page resulted in a prompt and correct result. On the off chance that the problem was on the home page, I tried hitting a different page and was successful. After logging in as admin (or any other authenticated user) I was able to browse to the home page with all appearing to be fine. But, after logging out, the home page again became inaccessible and a System.OutOfMemoryException was generated. Knowing that one major difference between an authenticated and unauthenticated user is that module Cache Time settings apply only to the unauthenticated user, I had my first clue of what to look for.
The problem was finally tracked down to an instance of the Events module in Event List mode configured to appear on all pages. On the home page only, the Cache Time of this module had been set to 300 seconds. All other pages (which functioned correctly for both unauthenticated and authenticated users) had the Cache Time of this module set to 0. After resetting the Cache Time of the event list on the home page back to 0, the site was back up.
As far as I recall, the Cache Time setting of the module on the home page had been set several months ago when the site was first set up. Could it be that the caching problem did not show itself until the number of events in the master Events module reached a critical number - in my case about 125?
After tracing the problem back to Events module caching, I found this related forum thread: http://www.dotnetnuke.com/Community/ForumsDotNetNuke/tabid/795/forumid/20/threadid/32783/threadpage/1/scope/posts/Default.aspx