In other words, have you installed any anti-virus (or maybe pop-up blocker - but i seriously doubt it) firewall thing? I had the same problem for a little while when I had Norton Antivirus 2002.
Every time my IP address changes (which has only happened three times since I've had this cable service) I have to log back in as well. I run a YaBB myself, and have noticed this behavior on that board as well.
Are you on dialup, or does your ISP regularly change your IP address on you? If you don't know how to check this, go to http://www.whatismyip.com. If the number it reports is different at different times (e.g. between logins), this might be the source of your problem.