AVCHD clips you transcode to ProRes from the Log and Transfer window are sometimes jumpy or distorted.
The solution from Apple is as follows:
This may happen if you installed the Perian QuickTime component. To temporarily disable Perian while using Final Cut Pro:
Open System Preferences.
Click the Perian icon at the bottom of the System Preferences window.
Select the General tab in the Perian preference pane.
Click Remove Perian.
If you would like to enable Perian after you finish working with your AVCHD clips, return to step four but click Install Perian to enable it.