503 Error Under Heavy Load

I have a Windows Server 2003 Enterprise (32 bits) running IIS 6.0, with ASP.Net 1.1.4322. Basically it's an application and image server, that has a large amount of concurrent accesses.

Around 800 users simultaneous access simulteneously and ASP.NET app, that will request some date from an MS-SQL Server database and point the browser to image file (all at once), every few seconds (it's a live webcast applications). The server is quite robust, with 500 GB in a RAID-5, 4 GB RAM, Dual Opteron Processor.

When the users access it, the machine memory and CPU usage a low. Also it has plenty of bandwidth, since it's directly tied to a Gigabit port on the datacenter. I kept an eye on all paramenters and there is no imporant bottleneck on the hardware side.

Here is my problem: after some "images" are sent, with 800 users, the users start getting a 503 error, and can no longer access the images.

On IIS HTTPErr Logs, I found these errors to be "Timer_ConnectionIdle". I believe it has something to do with the way the Application Pool is handling all those concurrent connections. No references to it are made on Windows Logs.

I've already tried tweaking all those paramenters on the Application Pool, but no results. For example, I tried increasing the Request Queue Limit value, but that doesn't seem to make any difference.

Also, I inserted "MaxUserPort=65534" on TCP configurations on regedit. Rebooted. Also, tried turning off keep-alive and decreasing connection timeout, then a restart of the w3svc (see [url]for the explanation about this). Tried changing each parameter individually and tested. No big difference after each modification.

Has anyone ever seen this? Is there a way to get this solved? Again, this address [url] has a description of what seems to be my problem... but I didn't manage to get this solved.