Troubleshooting 404 errors

We are encountering wierd 404 errors at random. When we restart the web server, these errors go away for some days and then they start appearing until we restart web servers.

I am not sure how to troubleshoot these errors. Enabled failed request tracing, but it does not show any useful debug information or at least I do not know what to look in the failed request log.

Here is the error message

ModuleName IIS Web Core

Notification 16

HttpStatus 404

HttpReason Not Found

HttpSubStatus 0

ErrorCode 2147942402

ConfigExceptionInfo

Notification MAP_REQUEST_HANDLER

ErrorCode The system cannot find the file specified. (0x80070002)

We added a route that maps the URL to a controller. 

 RouteTable.Routes.MapRoute("Forced lower case VFS", // Route name"vfslow/{*path}",
                new { controller = "Vfs", action = "GetFileLower" }
            );

When the URL of form ttps://cloud.centrify.com:443/vfslow/lib/ui/Ext4.1/src/layout/container/Absolute.js, GetFileLower method of VFS controller is invoked with path "ib/ui/Ext4.1/src/layout/container/Absolute.js"

This works most of the time, but when encountering 404 errors, it does not come into the controller, as if IIS modules are returning 404 errors right away.

Here is the signature of GetFileLower method:

[Anonymous]        
 public ActionResult GetFileLower(string path)
 {
   return GetFile(path.ToLower());
 }

Any ideas on how to debug this?

Here is the path to failed request tracing file with XSL file:  https://skydrive.live.com/redir?resid=F825B9A71458F626!155

Thanks in advance

Anil Lingamallu