You should also Repair Disk Permissions after trashing. However, when FCP goes crazy, I trash prefs, then run Disk Warrior, and finally, Repair Permissions. This has saved my bacon countless times over the years.
MacPro 8-core 2.8GHz, 16 GB RAM, OS 10.8.4, QT10.1, Kona 3, Dual Cinema 23, ATI Radeon HD 5870, 24" TV-Logic Monitor, ATTO ExpressSAS R380 RAID Adapter, PDE enclosure with 8-drive 6TB RAID 5
FCS 3 (FCP 7.0.3, Motion 4.0.3, Comp 3.5.3, DVDSP 4.2.2, Color 1.5.3)
FCP-X 10.0.9, Motion 5.0.7, Compressor 4.0.7
Pro Tools HD 10 w SYNC IO & 192 Digital I/O, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec DSP Monitors, Prima CDQ120 ISDN