Hi,
I had exactly the same problem. And I tried everything, erasing the preferences and plists of FCP. Then I heard it’s a problem with the early versions of FX Factory Plug ins, but also uninstalling those did not help. The only solution to this strange bug is to completely reinstall your whole system. To avoid that, I logged and captured all the material on a friend’s Mac, where it just worked fine and then took the material back on my HD. Sucks, but seemed to be the only solution, when you want to avoid the hustle of rebooting your whole system.
Greetz.