"The web server connection was closed" error

Hi,
I am having a problem with my website which is on IIS 6.0. When I login to my website internally(login from ie which is on my webserver), everything works fine. But sometimes, when I login to my website externally(login from ie any other computer), I am getting the following error:

The page cannot be displayed  
 
Explanation: The Web server connection was closed.
 

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Try the following:

Refresh page: Search for the page again by clicking the Refresh button. The timeout may have occurred due to Internet congestion.
Check spelling: Check that you typed the Web page address correctly. The address may have been mistyped.
Access from a link: If there is a link to the page you are looking for, try accessing the page from that link.
Contact website: You may want to contact the website administrator to make sure the Web page still exists. You can do this by using the e-mail address or phone number listed on the website home page.

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Technical Information (for support personnel)

Error Code 64: Host not available
Background: The connection to the Web server was lost.
 

I was wondering if somebody else has this problem before. Thank you. (I have tried to set EnablePMTUDiscovery bit to 1, it didn't help.)