Apple has posted a troubleshooting basics document:
http://support.apple.com/kb/TS3893
Seems like a few steps you've already done, a few more possible.
4. Make sure ProApps QT codecs are installed
7. Does this Library work in another user account? Use a copy of the Library!!!
10. Reset FCPX prefs- there are several prefs that could be affecting the app that need to be addressed individually.
11. Remove and Reinstall FCPX
These are numbered by the steps in the above article, where the instruction are for doing each step.
Good hunting!
PS Sorry about the "old chap" in a previous post, I had watched a bit of Monty Python and was using my british army general voice when I typed it. ::grin::