I'm on Panther 10.3.1 on 2 Macs and get the same thing on both. Sometimes I go to a menu and click, the name goes black but the menu never comes down - same for menus on the bookmarks bar as well as the main top menu. Only cure is to quit and relaunch.
Also sometimes it just stops working - going to any site it just sits there doing nothing. Yet flipping to IE proves my connection is fine. Again, only cure is to quit and relaunch.
All very odd...