Mark, what's the value of your servers "max_input_vars" setting?
It's possible, with a lot of forums, that your reaching this setting and that's why things are zero'ing out. Try inserting "php_value max_input_vars 5000" into a .htaccess file under your UBB.threads install and check your phpinfo output to see if your host allows you to offset, and if so try to recreate this bug.
I have an entry on the Wiki for this:
Issues with the PHP 'max_input_vars' Setting