Does it happen almost exactly once a month?
I saw this for the first time yesterday. I was logged in when I refreshed the main index sometime during the morning (GMT) yesterday, but when I refreshed the list again in the early evening, I was logged out.

Might be a bug with the cookies implementation - you could keep an eye on the expiry date/time (Tools->Cookie Manager->Manage Cookies in Netscape 7) but it could be a server-side problem.