I am having issues with session loss. This is on a Windows 2008 r2 server with iis 7.5. The sessions are stored 'in-proc'.
I can't reproduce the problem on demand, but it seems to occur for about 30 minutes to an hour around the same time of day when it happens (doesn't happen on the same days). When the problem happens, the sessions can be lost in a few seconds to several minutes.
In the past I had session loss problems, but this was related to the app pool being recycled. I fixed this issue by changing the recycling settings in IIS (I have not changed the default value in web.config). I don't believe this problem is caused the the app pool recycling, because when the issue begins, i open task manager on the server and watch the worker process. The PID never changes and the cpu / memory use stays within a reasonable range.
I thought the issue might be related to the anti-virus software, but the scheduled scan time is not during the time when i typically see this problem occurring.
I also checked to make sure the web.config, files in the app folders are not being changed by someone. I check the date modified value on those files and those times don't coincide with the session loss.