Menus are disappearing

Hello, the top menus on our website keeps disappearing after scrolling down. Once you scroll down on a page that has submenus, then when you go back up to the menu, they start to disappear as you put your mouse over them.

I have 3 screen shots showing; full menu, them partial menu as I use mouse over it & then missing menu as I leave the menu with the mouse, but I do not see an upload button (attachment)???

My site is http://www.medicinewheelconference.org/conference-registration/

What can we do to stop this from happening?