The audio/video sync problems do happens fairly often to me. What I do is simply try do edit the audio where it goes off course, to re-sync it. It might be a better way out there though.
I don't know much about "MPEG Streamclip", but if you want to try an other program and see if the clip is in better sync, I can recommend DVD Decrypter. ADigitalMan has a wonderful guide on using it right on this board.