Web Application runs too slow

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IIS6

ASP.NET

SQL

IIS

server

Dear members,

I have an windows server 2003 and sql server 2008 express edition.
Three asp.net web applications are running on same server.
Previously it used to run perfectly
Suddenly one of three web application runs too slow after certain interval.
Its takes almost 15 to 20 sec to perform server events and the end user keeps on waiting.
To overcome the problem ,i always reset the iis, and for time being the problem gets solved and application runs smoothly.

As far as application is concerned,code is optimised and no such issues with records retrieval from database.

What measures can be taken to speed up the iis , or else how could i find out whether its a iis problem or not
I need to find the pain area?