Why does debugging keep timing out in IIS7?

http://weblogs.asp.net/soever/archive/2009/06/18/debugging-sharepoint-asp-net-code-smart-key-codes-disable-timeout.aspx

Your App Pool -> Advanced Settings -> Ping Enabled to False

When you are debugging, IIS will not service any other requests until you are done stepping through your code. That includes the "ping" request that IIS sends to itself. Since IIS doesn't hear back from itself, it decides to shut itself down, which promptly terminates your debugging.

The solution is to increase the Ping Maximum Response Time in the application pool settings from its default value of 90 seconds. Set it to something high enough that will give you enough time to debug your code (like maybe 300 seconds).

Microsoft has a long-winded write-up here.

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Edit: Others have suggested setting "Ping Enabled" to false. There are several reasons why I prefer to keep it in place, just with a larger interval, but the most important is that you will (most likely) have worker processing pinging enabled on production, and you should strive to develop and debug under a configuration that is as close to production as possible. If you do NOT have ping enabled on production, then by all means disable it locally as well.