Displays preferences don't stick for monitor arrangement after upgrading

I have three monitors on a MacPro... one is a TV on the wall, two are late-model Apple displays on my desk.

I have never had a problem with the OS remembering the arrangement of these three monitors, but ever since upgrading to Mountain Lion, the two monitors on my desk are constantly getting switched up after each restart... every time I restart my mac, I inevitably have to rearrange the desktop monitors to the correct position and move the menu-bar to the correct location.

This is exacerbated by the fact that the third monitor sometimes is/isn't switched on. Before Mountain Lion, it didn't matter if the third monitor was on -- I could turn it on/off at any point with the mac turned on or off... and my monitor positions were always preserved. But now, I am constantly having to rearrange the monitors, sometimes with the third one on/off -- and reset the monitor arrangement from scratch, almost on a daily basis.

Any ideas out there why my arrangement prefs won't stick?

Thanks in advance.