Video not in sync with audio - Final Cut Pro

Hey Guys,

I am editing a video in FCP and all I am trying to do is take two different videos of a radio interview, sync them up with the actual radio interview audio as opposed to using the video camera audio, and then cut back and forth between the two videos (each containing a different perspective of the interview). I have synced up the audio with the camera audio, but then halfway through the video, the footage and audio are no longer in sync. the weird part is that the waveforms still line up... my audio clip isn't out of sync with the original video audio, the video audio is no longer in sync with the video... is that even possible? Is it a rendering/lag issue, or would the video remain that way when bounced out as a final product? Im a musician and an audio engineer, video editing is not my forte and Im frankly a little lost with even a simple editing project. Can anyone offer perspective/assistance. Thank you!

2011 macbook pro

2.7 GHz Intel Core i7

memory:16 GB 1333 MHz DDR3

Intel HD Graphics 3000 512 MB

FCP: 7.0.3