Intermittent IIS Connection close issue

really odd one here.

I have a website which runs on IIS fine most of the time. On some occasions though I get a white 404 page with the text:

Not Found HTTP Error 404. The requested resource is not found.

Running fiddler I can see the following http headers:

HTTP/1.1 404 Not Found Content-Type: text/html; charset=us-ascii Server: Microsoft-HTTPAPI/2.0 Date: Thu, 10 Nov 2011 21:25:01 GMT Connection: close Content-Length: 315

So a "connection close" error seems to be happening intermittently.

A couple of refreshes later the page loads fine with the following headers:

HTTP/1.1 200 OK Content-Type: text/html Last-Modified: Thu, 10 Nov 2011 15:53:56 GMT Accept-Ranges: bytes ETag: "02d1f3c09fcc1:0" Server: Microsoft-IIS/7.5 X-Powered-By: ASP.NET mycustomheader: 01 Date: Thu, 10 Nov 2011 21:25:24 GMT Content-Length: 16307

I notice that the 404 header doesn't have custom http header response which I've added as part of my debug steps. In IIS if an error is being served by a specific web should it not have the custom header aswell?

Anyone know why certain http requests would intermittently be closed by IIS?

Thanks,

Ed