I encountered an issue recently where Final Cut Pro will jitter and glitch out every half a second or so when importing AVCHD footage from a Sony HD camcorder.

I eventually tracked down the error thanks to some google searching and it turns out that Perian is the culprit.

Apple even have a couple of knowledge base articles about it:
Final Cut Pro/Express: Ingested AVCHD clips are sometimes distorted
Final Cut Pro, Final Cut Express: Unexpected quit during AVCHD ingest

If you disable Perian in the System Preferences before launching Final Cut Pro, it’s all good.