Hi
I have tried them

Modification in Machine.Config, Web.Config or Global.asax files
Application's bin directory or its contents modified 
Antivirus is running on the  .config /.aspx files
The number of re-compilations (aspx, ascx or asax) exceeds the limit specified by the <compilation numRecompilesBeforeAppRestart=/> setting in machine.config or web.config  (by default this is set to 15) 
The physical path of the virtual directory is modified 
The CAS policy is modified
Sub-Directories of Application are deleted or renamed

nothing changed

App domain restarts

will check on it

"System.Web.HttpException: Request timed out."

not getting this error

<sessionState cookieless="false" timeout="330"/>

session expired randomly not happen so often, but still happen
In fact, we have set this programmatically at Session_Start event

memory recycling

Disabled

Worker process recycled in min

Default, 17xx mins

Antivirus

Not installed

This issue happen even to a laptop at support site, but not in our testing environment

Any ideas?

Thank you