Automatic log out at random intervals for single user

A user in our Extranet SharePoint FBA based site gets automatically signed out after random intervals. For all other users, it is working fine.

The IIS time out is set to 120 seconds. Checked that IE user authentication settings in user’s machine is set to automatic logon with current username and password; cache and cookies are deleted. Found that issue exists in multiple machines. So, it seems to be a user account related problem. Removed and re-added user in extranet site. But issue is not resolved. Any ideas why is it happening and what could be done to solve it?

I'm a site collection administrator on the extranet site with no access on farm.

Thanks.