User experience spontaneous logouts

One of the users of my RoR website, mushroomobserver.org, has been complaining about getting spontaneously kicked out the site. I've looked at the logs and over the last 8 days this particular user has been logged out 15 times. Interestingly most of these logouts are within a second or two of a regular page load. My only guess is that he has some sort of malware on his system that is generating these requests. None of my other users are having this problem. Has anyone else ever see anything like this? I tried some google searches, but didn't find anything. Assuming this isn't a RoR specific problem, I'd be happy to get any recommendations of other sites or mailing lists where I might find some help.

Thank in advance, -Nathan