Menu items rearrange themselves when I post a new node

In my test site, the menu items keep getting jumbled. In admin/build/menus, I fix them and they are correct for a while. But as soon as I post a new node, they get scrambled again. This happens even if the new node is something like a blog post that isn't being assigned to a menu.

I suspected that they were all getting bunched up in the -50 to -47 range, so I turned off Javascript in my browser and carefully reassigned them manually to spread-out numbers in the -48 to -20 range. That did the trick... until I posted a new node again.

When you look at the list of menu items with Javascript turned off, the weird thing is that all the out-of-order menu items have been reassigned to -50 and yet they're at the *bottom* of the page. So the list is like this:

Correctly ordered item: -47
Correctly ordered item: -42
Disabled items: -30, -12, 0, 5, etc.
Incorrectly ordered item: -50
Incorrectly ordered item: -50
Incorrectly ordered item: -50

Has anyone ever observed this?