My guess is you have an error in the way you set up the menu. The list shouldn't disappear as you move the mouse down. Check the distance between the main menu and the pop up menu. You probably have a slight gap of a pixel or two and as the mouse moves into that gap the menu disappears because you are no longer hovering over it.
You don't need to build in a delay for that, you just need to tweak the CSS and find the error.