Why Oh Why

Does anyone know how to fix this issue I'm having?  It seems that on a semi-regular but not exactly scheduled basis, I get logged out of almost all the sites I'm logged into.  This affects message boards such as OS and a few others all the time, LJ some of the time, and Yahoo and Google rarely.  I've checked my settings and control panel and I can't see how to fix it at all.  I don't delete my cookies or clear personal data or any of that stuff, and this is a new issue since I got the Vista + Firefox 3 combo.  It never happened like this on my XP machine, and it never happened when I used the Vista laptop with FF2.  My inclination is to think it's a FF3 issue, but I don't know how to change it.  Can anyone help?