can't move mouse over admin block

great idea... but cannot implement successfully on my site

the admin bar appears on installation... when I move mouse over a menu item, drop-down menu appears, but offset to the right of the menu item... as soon as I move mouse towards drop-down menu items, by moving off the top-menu item, the drop-down menu disappears!!

So frustrating! (There is about a single pixel diagonal "bridge" between the admin menu and the drop-down menu that follow mouse hover, but I've only ever managed to "cross it" to the d/d menu once!

Is this a bug? Or have I installed incorrectly somewhow?

thanks!