The submenu that displays off the main nav menu has problems. Sometimes it is impossible to click the submenu because it disappears.
I have set the menu to two lines and the top row of items cannot use the submenu at all because they are wiped out before you get to them.
Also - some submenus become the default and display all the time whether the main item is selected or not.
In short - the current system is full of bugs - please fix it!