I had that exact same problem.
As I had the inside panel off of the door at the time  it was real easy to
dismantel the entire latch mechanism and clean and inspect it.
As it turns out the problem was wear in the latch mechanism inside the
door lock itself(at the rear of the door).
Clean every thing you can see real good- then work all the parts you can
observe and see how they interact- my permanent (so far) solution was
simply to take a small file to the lock parts where they interact and
clean them up a bit.
Of course the quick fix would be to buy a new latch mechanism but I dont
think you can do the R&R without taking the door panel off.

Jaymac,  87 Vanagon GL (2WD) 195,000 miles  (20,000 miles on motor)

sam pacifico wrote:

> The sliding door on my '87 vanagon has suddenly started to pop open on
> its own while the vehicle is moving.  Is there some adjustment that I
> can make to secure this door again?