Sync video and imported audio in imovie

I have a video with audio on tape transferred to digital. The sound is not good quality. I also have a on cd the audio recorded by an engineer. The tracks are spaced, so are being synced track by track. Some of the video has places where it goes black or pixelates(?) and the sound is garbled. I am trying to sync the video and the separate audio(from cd) but am having trouble where the video drops out. I can sync the beginning of the video but don't know quite what to do when following video dropouts occur. Any pointers is much appreciated.