"Request timed out" error before configured timeout elapses

ASP.Net 2.0 page returns error "HttpException: Request timed out" after
varing amount of time (anything between 3 and 30 min) when uploading a big
file to it. executionTimeout in web.config is set to 20000 (5.5 hours),
response.write(server.scripttimeout) does return 20000, which confirms that
the setting is in place. maxRequestLength is set to the number big enough to
allow the file through - uploading the file over local network when the whole
process takes a few seconds does work. the server has plenty of memory
availbale - 4GB total while consupmtion sits around 1.8GB

here are some details of the error from windows event log:

-------------------------------------------------------
source: ASP.NET 2.0.50727.0
category: Web Event
event id: 1309

Event code: 3001
Event message: The request has been aborted.

Process information:
Process ID: 6992
Process name: w3wp.exe
Account name: NT AUTHORITY\NETWORK SERVICE

Exception information:
Exception type: HttpException
Exception message: Request timed out.
------------------------------------------------------------

thank you in advance for any help

andrew