Question operation timed out

[reedone816]

hi, hope someone can help me...
here is my configuration:
windows2003, IIS 6 with public ip and domain name, VS 2008 (asp.net/vb.net), using comodo firewall.
I have a page where it always return this error:

Network Error (tcp_error)

A communication error occurred: "Operation timed out"

The Web Server may be down, too busy, or experiencing other problems preventing it from responding to requests. You may wish to try again at a later time.

For assistance, contact your network support team.

this webpage only return this error when I use different ISP than the web server, when I use the same ISP as the webserver or running locally in webserver it returns the result.
and even using the different ISP, another webpage that is the same report webpage like the error one is able to show result.
this error error only happened this several months, before it is ok even though it is running for several minutes.
the bad thing is that I can't remember what was changed before...