If for some weird reason your menu is not saving properly and it’s being cut after some items. Is most probably that you recently upgraded PHP to 5.3.9 or above.
Recently one of my clients which uses Goddady hosting (puajj I hate Godaddy) upgrade from PHP 5.2 to PHP 5.3.
After some time he tried to add one more item to the wp nav menu (a big site with at least 100 menu items) and suddenly half of his menu disappeared!!!
After some time and reading some forums I realized that the new PHP comes with a max_vars setting that could affect WordPress menu and some other things.
So to fix this I simple created a php5.ini file and placed in the root of his directory (It will probably change for other hostings)
In this file I just entered max_input_vars= 5000 . The whole file looks like:
post_max_size = 50M
upload_max_filesize = 50M
memory_limit = 256M
Hope it helps!