drop down menu bug

My site uses nested drop down menus, that is each menu item has a set of submenus and each submenu has its own set of submenus. At the first and third level, they work fine. But if you place the cursor on the second or fourth level and hold it motionless for more than a second, when you move the cursor again, the drop down menus disappear and you have to start all over again from the top menu item.