Problem: Application Pool restart.

We have a Web Application / Service running on IIS 6.0 on ASP.Net 2.0
framework. The service performs some operations in worker threads. At
times the Service stops suddenly and so do the worker threads.

For our application to function, we want the service to be up and
running without interruption. We changed all the recycle settings; but
the application pool still recycles after an indefinite time.

Here are the environment details / settings:
Operating System: Windows Server 2003 Server Edition
.Net framework version: 2.0.50727.42
ASP.Net Version: 2.0.50727

The settings on the Application pool are:
Recycling
Recycle worker processes (in minutes): false
Recycle worker processes (number of requests): false
Recycle worker processes at the following times: none
Memory recycle: no

Performance
Idle timeout: false

Health
Enable pining: true
Enable rapid-fail protection: true

Identity: Local System

If anyone can help in this regard, I want to know why the application
pool restarts and is there any way to stop it?

Thanks and Regards,
Tushar