I am having the same issue with FCP on my machine tonight. I’ve never had jumpy playback before tonight. I played the clips both from my internal HD and an external HD with the same problem. The QT clips play smooth outside of FCP. I did download some new codecs today trying to get an AVI file to convert properly in QT Pro, but I ended up not keeping any of them. Could this be related to the playback in FCP? I appreciate your help troubleshooting this.