have you tried setting the Document timeout to a lower value than default (Default=480 minutes; try something like 180)?
This setting govern how long QVS will keep a file in memory when it is idle.
I generally use 180 minutes. After 180 minutes of inactivity, your user is most likely done with your QV file for the day; Or your qvw is so awesome, that your users just love staring at it. ;-)
You can find this in QEMC --> System -> Qlikview Server --> [Your Server] --> Documents
I guess this has nothing to do with your w3wpissue though ... maybe it still helps... Regarding w3wp, have you tried this --> http://support.microsoft.com/kb/916984