I thought to do a
watch -d 'ls -l' on the file MythTV was creating when it was recording a show this evening and what I saw was very revealing. There seemed to be gaps of about 5 to 10 seconds where the file size did not change.
I’m using the XFS file system for my MythTV recordings, as it is better suited to the large files common to digital video. I did upgrade the system’s kernel, which may have somehow broken the XFS driver.
I have another partition formatted in EXT3. My next test will be to send the files to that partition and see if they continue to miss frames.