Appearance>Menus problem

My theme supports wp-menu but I am having trouble when saving the categories entered.
I have an extensive menu created with over 150 categories and sub-categories. I am able to create and save the menu on the first attempt but whenever I need to make a change or an addition, I am not able to save the updated menu. I get what seems to be a “time-out,” and then /wp-admin/nav-menus.php turns blank.