Hi Ellis,
Since this seems to be a case of the database losing, or losing track of, records, my first instinct would be to optimize the catalog (File > Optimize Catalog). Next I would try to see if there's a pattern to the "lost" files (files related to a folder moved/renamed, files renamed, etc.). Then you might try synchronizing the volume in Lightroom so that you get a full re-scan of the drive, and then examine the pattern of files being re-imported to identify a common factor. Finally, although the image files themselves seem to be intact, you might want to check the directory structure of that volume, using Disk Utility > First Aid > Repair Disk, or, even better, using the Volume Structures test in Tech Tool Pro 5.