soundtrack mix out of sync w/ final cut timeline

Hello folks,
Well, I'm fairly stumped here. I'm working on an FCP video project and using STP to master the sound. My workflow is: rough-edit levels on FCP timeline, export to multi-track project in STP, master sound in STP and export mix, insert exported mix back into FCP timeline. My problem is: the final mix continues to come in and out of sync with the original video--some clip and interview audio will be perfectly in sync, others will be off by two frames or a fraction of a frame, still others are off by much more. And throughout the mix this problem comes and goes, even for audio clips that live in the same track. All of the original audio files in the FCP timeline are perfectly synced, and if I even just export the project to FCP and then run a mix (without doing any mastering in STP), the problem still emerges in the exported mix. I really can't figure it and would greatly appreciate any help that can be offered towards understanding what's going on. Thanks much, and please let me know if I can further clarify the problem at hand.

Regards,
Chris Gallaway