I\'ve got vs2010 and vs2012 installed side by side. If I open up our MVC site in vs2010 and run it using the development web server it works fine, if I do the same thing in vs2
You have to raise the concurrent request limit. Read the following :
Modifying the ASP.NET Request Queue Limit http://technet.microsoft.com/en-us/library/dd425294%28office.13%29.aspx