I am having the exact same problem. I have completely removed FCP per the online instructions (deleting pref., etc...), ran the extended diagnostics on my hardware, and still can't figure out why it's doing this. Is yours still giving you this trouble? I am running FCP 10.0.8 on a 15" MBP Retina on OS 10.8.3 with my projects on an external FW800 HDD.
Anyone's help with this issue would be greatly appreciated!