Error after 5 minutes of processing

We have a web site that allows customers to inquire price and stock for
parts. The customer enters the required parts in a ASP data grid bound to a
data table. When they complete entering parts and submit to the the server,
the server fetches the parts and price for the parts and displays in the grid
to the user.
This process works fine as long as the server completes the processing
within 5 minutes. It is able to display the price details in the grid
successfully. If the processing exceeds more than 5 minutes, the client exits
out with a message stating Page cannot be displayed or Cannot find server or
DNS Error.
We tried increasing all known parameters in machine.config and web.config
files without any success.
The site is a secured site and works under https://. It is protected by both
server and client certificates.
Does anyone have any idea of what to do? Kindly help out

Radha