I had the same problem when editing from a P2 card. I traced it down to the dvcpro 720 30p codec gets deleted from my hard drive after getting one of the final cut pro updates (not sure which one). Not sure how to resolve this other than formatting and reinstalling without doing any updates. Of course then all the projects you edited in final cut (updated version) will not open without the update.
I have tried downloading stuff from Panasonic and Apples coded compatibility and it does not work. the lip sync is way off. The original FCP disks have the codec and you can manually install them and then they disappear within a day or two????
Fun!
Good luck,
If anyone knows how to fix my problem, i would love to hear from you.
kind regards,
Scott