Have you tried trashing your preferences?
I can't promise anything but faults like this can often be fixed by clearing out FCP X's preferences (for some reason, they get knotted up from time to time).
Download Digital Rebellion's Preference Manager (free, simple to use, and quite safe).
http://www.digitalrebellion.com/prefman/
With Pref Manager, you can backup the Prefs first, then if trashing makes no difference, you can restore all your settings.
If you trash them and don't restore them, you will need to go over your FCP X settings again, so it's a great idea to backup Preferences from time to time when FCP X is working well, then you can ditch the bad prefs and restore from your (Digital Rebellion) backup.
It's worth a try.
Andy