The problem is likely one or more relations that are being lazily loaded rather than eagerly loaded. The good news is that you don't need the using
statement for a DbContext
as the context already manages the connections, leaving little need for the Dispose (cf, http://stephenwalther.com/archive/2008/08/20/asp-net-mvc-tip-34-dispose-of-your-datacontext-or-don-t.aspx). Alternatively, you can make sure that any relations that you have are eagerly loaded by setting the load options for the context.
Note: if you use IoC and inject the context rather than create it directly, you can avoid the problem entirely as the DI framework will take care of cleaning up the context for you.