Problems with the current navigation

Hello!
We have a problem with the current navigation on the site. Navigation disappears from time to time, which is located on the sixth level (assuming that theroot site is the first level). At the same time, the navigation on the fourth level continues to be displayed.
As we know, the disappearance of the current menu is not connected with our actions (for example, we used to think that it happens when you transfer the site). At the same time, if you go to "Site Actions", "navigation" on the site where navigation is broken, the current navigation is still displayed.
It helps to restart IIS, but it is a bad solution, and we hope very much for your help!