Inbox oddities

I use Thundebird, and something weird is going on with my default Inbox. I can't be sure, but it sticks in my mind that it started shortly after the last Windows XP update (KB887742), which I did Feb. 22. It now takes much longer to check for mail on the server - judging from the status bar at the bottom of the window, it seems to take several retries before it makes a connection. While the mail is being downloaded and shortly thereafter it shows a ridiculous number of unread messages in my Inbox - like 243! And although it is set to check for messages every 10 minutes, it no longer does. I have to click on Get Mail each time. Once the mail is downloaded I can double-click on Inbox and it will open a new Window with the correct number of unread messages. The old Window now also shows the correct number and it stays right until I close and reopen the app.

I have my GMail account set up to come into its own Inbox, and it doesn't exhibit these problems. I tried reinstalling Thunderbird - but I don't think I uninstalled first. At any rate it didn't help.

Has anyone else encountered anything like this? any ideas?