I've come up with a fairly straightforward workaround for this. In Final Cut Pro X, before sending to Compressor, do an audio-only export to an AC3 file (Share...Export media, select "Audio Only" for Export and "AC file" for Audio file format). Then use the "Send to Compressor" function from the Share menu in FCP X. In Compressor, create a DVD using the standard "Dolby Digital Professional" and "MPEG-2 for DVD" settings and adding a "Create DVD" job action. Make sure the target AC3 file name in Compressor has a different name (or location) than the AC3 file you created in FCP X. The resulting DVD does have intermittent audio as before, so it can be discarded.
Here's the trick:
Open the folder containing the Compressor-created AC3 file, copy the file name, then move that file to the Trash. Find the FCPX-created AC3 file, rename it with the name of the Compressor-created AC3 file, and move it into the folder where the Compressor-created AC3 file was before you moved it to the Trash. Now you can resubmit your job from Compressor and it will burn a new DVD using the AC3 file you created from FCP X, which should have good audio. For me this created a great looking DVD with the high quality MPEG-2 video from Compressor, and the good audio AC3 from FCP X.
I think there is a bug when sending FCP X projects with multi-cam portions to Compressor. I've seen two other cases of this on the discussion boards and multi-cam appears to be the common thread. I can't think of any logical reason that an AC3 file created in FCP X would sound fine, but the same FCP X project sent to Compressor results in an AC3 file with intermittent audio.
At one time I thought the issue was exceeding the capacity of the DVD. But after experimenting I learned that an AC3 file created in Compressor from my multi-cam FCP X project has intermittent audio, even without a "Create DVD" job action. (I used VLC to play back the AC3 file and had the same intermittent audio I had with the DVD).