Place the following in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\RPC
That key just has to be present. You don't need a value. Also, you will have to place that key in every registry that is running the Sandboxed Code Host Service. Once you do that, I would recommend that you restart the machine.
Here is a great blog article in regards to this issue. http://blogs.msdn.com/b/sharepointdev/archive/2011/02/08/error-the-sandboxed-code-execution-request-was-refused-because-the-sandboxed-code-host-service-was-too-busy-to-handle-the-request.aspx