FCPX freezes up along with the OS

In the last few days FCPX will lock up when I click on any item either in the browser or timeline. It also locks up the OS so that I am not able to Force Quit FCPX, the dock will not work either so I can't launch another program or even to get the Force Quit window to show up. I have to do a hard shut down.

The last time the computer locked up I was playing the timeline and the audio was heard but the video image stayed at the first frame. The only pieces of equipment hooked to the MacPro (trashcan) are a Promise Pegasus R6, two Apple 27" Thunderbolt monitors and some speakers connecting through a fiber optic cable to the computer.

I repaired permissions. The permissions that related to FCPX that were wrong was on the ProApps. I haven't trashed the preferences yet as Richard Taylor says that is the last thing to do if nothing else works.

I would disagree that trashing preferences is the last thing to do. It's not really drastic and in legacy FCP it was a regular occurrence. I have had my whole system lock up as you describe. I have yet to figure out what caused it and it hasn't done it in a couple of weeks. Sorry I can't say more than that but maybe it will "just go away" for you too. I try to keep all of my software updated at all times so maybe one of those updates fixed it.