"The page cannot be displayed because an internal server error has occurred", Azure Website,

I have deployed the azure website to East US location, Reserved instance. I have CustomErrors=On. After few minutes of work (with quite good performance) app is hanging, slowing down. (I don't see any errors in elmah.) After that starts to return 500 error. My custom error page is not displayed (I see only "The page cannot be displayed because an internal server error has occurred." in plain text) so I can't even customize it. Then the app is restarted. We cannot go into production like that. I don't know what to do.