Several folks suggested cleaning and relubing the door hinges, tracks,
and mechanism. The door now moves a little more freely but still sticks
occasionally. I haven't gotten around to tinkering with the door this
week since the wife drives Wolfie to school every day and I drive to
Pasadena for work. This weekend I will look into other causes and keep
everyone posted.
To those who gave me suggestions, thanks for spending the time.

Steve Bibler
1990 Vanagon, "Wolfie of the sticking sliding door"