Individual passwords get randomly dropped

Although I posted in the "forgot all passwords" threads, that's not really my problem. For me, I find that Firefox has occasionally forgotten a password to a particular site, forcing me to re-enter it. There is no pattern to this, sometimes it's a site I have not used in months, sometimes it's one I logged into just the previous day. Other passwords are still there and fine. I'd say I run into this 2-3 times a week. I have hundreds of passwords saved.

I've taken to recording my passwords in another application as a backup, because I can't trust Xmarks to remember them. I do have several PCs sharing the same Xmarks profile, but typically I do not separate out passwords by profile.

This started happening long before FF3.5 was released.