bad audio/video syncing in final cut X

Hi, I'm trying to sync footage of a Canon 7D with audio from a zoom 4n. I select the audio and video and press syncronize clips. Then I get a clip with two audios. It works fine so far, the problem is that as time pass, the clips start getting out of sync. It starts well, but after a few seconds I can see that the audio clip is going faster than the audio from the camera, and after 10 or 20 seconds the audio of the zoom is offset by 1 or 2 seconds. Both audio are 48 khz. I synced both of them manually in Final Cut 7, and I dont have this issue, they stay synced.

Any help?

Thank you very much