Page cannot be displayed after about 15 minutes

Hi, I have a asp.net 4.0 application. A pop-up window has to do some calculations which run for 1 minutes to about 30 minutes. In one instance where it would come back for some 20 minutes.ie displays "page cannot be displayed" on production. where as I get the response properly if I run the same input from my local environment. What count be wrong? I know that the httpRuntime ExecutionTimeout is set to 1hr. Thanks in advance for any advise on this. Manju.