Navigating to sub-menus can be frustrating :
One solution is to add a delay to give enough time to the user to move the cursor to the sub-menu before closing it.
But the resulting "laggy" effect can also be frustrating.
A better solution is to implement something called directional menu aiming :
This is done by detecting whether the cursor is moving near vertically (between menu items) or in a triangular zone (toward a sub-menu) :
This technique is used for example on Amazon's main left menu:
http://www.amazon.com/Please check this link for a very nice explanation:
http://bjk5.com/post/44698559168/breaki ... a-dropdown