Some of our menu items have disappeared off the main nav menu bar, and when I try to add them back they either won’t add or something else disappears.
I thought it was a plug-in incompatibility, but I’ve gone through and even when all plug-ins are disabled it still won’t let me add any new items without something else disappearing.
Is there a limit to the number of menu items that I didn’t know about?
Hello there,
Wow, you have really a lot of menu items, congratulations 
Number of menu items is limited by server resources, so you will have to contact your hosting service provider and to tell them about the problem. They have to inspect error log file, and they should know how to solve this after that.
Probably it is problem with PHP directive max_input_vars, and they will have to increase it. If they are using Suhosin, they will also have to increase appropriate directives.
You can read more about this problem here http://sevenspark.com/wordpress/menu-item-limit
Best Regards