Troubleshooting, step one.
Deleting FCP Preferences:
Open a Finder window. Set the view mode to columns:
Click the house icon that has your name next to it.
Click the folder "Library" in the next column.
Find and click the folder "Preferences" in the next column.
Find and click the folder "Final Cut Pro User Data" in the next column.
Move the files "Final Cut Pro 6.0 Prefs", "Final Cut Pro Obj Cache" and "Final Cut Pro Prof Cache" to the Trash and empty it.
When you have a stable set of preference files, download [Preference Manager|http://www.digitalrebellion.com/pref_man.htm] to back them up. If you have further problems, the backup can be recalled and you can continue working.