Primary navigation setting — not saving

With WP and the Twenty Ten theme, there are two ways to update the nav menu: you can use the native dashboard tool (Appearance > Menus) or you can customize the theme.

When I try to make a change using Appearance > Menus and then save, all the top level pages in my site appear in the nav bar. When I go back to the settings again, no primary nav menu is selected. When I select it again, it restores the menu but the menu does not include the last round of changes I made before it looked all weird.

I’ve seen older posts like this, and I’ve experimented with a lot techniques that might potentially fix it with no luck. I’ve also burned incense and prayed to the technology gods, but none of these measures has worked. Anyone know why the nav menu gets corrupted like this? I’m running the latest WP version and theme version. I haven’t made any site changes before this started happening. My nav menu is missing lots of items that used to be there, and I can’t seem to restore it.

http://masonnh.us