Inner menu hides, when mouse cursor moves on it

When I move cursor to the menu element, it opens and show menu. When I move cursor out of this element, menu disappears. This is normal behavior. When menu is opened and I move cursor to that menu (http://joxi.ru/uploads/prod/20130522/9c0/bad/83d631603fb9e73c4da28f7b5452453964959329.png) it should not disappear. But in some situations it happens. This is reproducible, just may be several menu traversing required to catch this situation.
Seems there is some problem with mouse area of menu element losing mouse when mouse area of menu has not mouse yet. If I move mouse very fast it works always, but when I move mouse slowly menu often disappears.