I have an IIS7.5 web-site, on Windows Server 2008, with an ASP.NET MVC2 web-site deployed to it. The website was built in Visual Studio 2008, targeting .NET 3.5, and IIS 5.1
Apparently this can have many different causes.
For us, the problem was that the DNS entry was configured for two IP addresses, but the IIS configuration would only listen to one of them. So we got unpredictable results, sometimes it would work, sometimes a few files (css, etc) would not load, and sometimes the whole page would not load.