IIS 8 Random 503 service unavailable

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http://serverfault.com – We migrated a busy website to Windows Server 2012 with IIS 8. The website randomly gives the error "503 service unavailable" after an user presses F5 the error is gone again. The website is build in ASP.NET MVC 3. The website runs on one application pool, with default settings There are around 500 to 900 concurrent users on the website during the day, and the error happens more often when there are more 650 users. The CPU and the memory use on the server is stable. There is nothing about the 503 errors in the application log, the IIS log and the event log. Does anyone has any clue what the p (HowTos)