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