nav-menus.php hangs at around 50 pages?

I’ve noticed that nav-menus.php takes longer and longer to update with every new page added to the navigation menu. For example, with just one page as a menu item, nav-menus.php saves/updates in about 3 seconds. With two menu pages as items, nav-menus.php updates in about 5 seconds, etc etc.

It was never a problem on my previous smaller sites, but I’ve now discovered that at just around 45 pages, nav-menu.php starts to get unreliable, and at around 50, nav-menus.php hangs altogether and will not save any new change. After I hit the save button, the page thinks and thinks for anywhere between 20 and 40 seconds, then returns a blank page without saving the changes. As such, I cannot continue building the navigation menu, and thus cannot continue building my site.

I have searched and searched and found only a couple other posters with similar-sounding problems, and only with previous releases of WordPress (I’m using 3.2.1). Their various suggested hacks to nav-menus.php do not seem applicable in this release. I’ve removed all plugins and gone back to the twenty-ten theme with no improvement.

The web server administrator does not see any out-of-memory errors originating from that page, and allows PHP a hefty 256MB memory limit anyway. I tried artificially setting a higher memory limit in wp-config, but saw no change. I seem to be stuck.

Please help!