On further exploration, the problem I am experiencing may be related to disk retrieval. The disk is a USB (i.e. slow) drive. Under load it seems to slow down sufficiently to cause vlc to miss data it needs, or so I think. Under tools, preferences, all, input/codecs, scroll down to "Advanced", I set File Caching (ms) and Disc Caching (ms) up from 300ms to 20000ms. That *seems* to improve things, but it's hard to know if it is better, or it is just coincidence.
JR