Occasional issues on syncing audio and video

At our college we have a room of 16 iMacs running Final Cut Express. We capture from DV on server and do the edit locally. Occasionally we have noticed a weird behaviour when the audio track lags couple of frames behind the video even though the tracks in the edit view are in sync. Also the captured material has audio track in perfect sync with the video so the problem is only when editing it. Has anyone noticed similar behaviour and is there any other cure than razor blading to fix it every time?