Your instance is unresponsive. When you look at your server.log, you see the following error:
02.07.2007 10:34:14 *WARN * servletengine: Thread pool limit reached.
To understand what is going on in your system, take a couple thread dumps (when you have the message in your server.log). When you analyze the thread dump, you can see that the most likely the threads are similar to each other. (For example, the threads are trying to access the same synchronized block or trying to lock an object.) Analysis of the thread dump can indicate that it is either an application or a product issue. Or, it could that many users are accessing your website (a noncached page) at the same time.