Scrollable submenu

Hi,

I have a lot of submenu-items, the list is longer then the height of my window. After searching this forum I found the next code:

.main_menu .menu ul {
height: 500px;
overflow: show;
overflow-y: scroll;
}

This is working fine, but of course it sets the height of all menu-items. In the original code the overflow was set to hidden, but then the code didn’t work.

Can you help me out with this please?
The url of my site is in the private section.

Regards,

Erwin