It seems that the list members who quickly guessed my problem to be the Hall
sender were correct.  After swapping out parts including an ECU, I finally
got my mechanic to put in a new distributor and that seems to have solved
the problem.  He basically refused to put in a rebuilt distributor, telling
me horror story after horror story about rebuilt items.  He also insisted
that you could not buy a Hall sender separately even after I printed out the
page from one of our vendor's catalogs showing a new Hall sender in stock
for my model.  All's well that ends well, I guess.  My van hasn't missed a
beat for a couple of weeks now [he said, gently knocking on the table].

Bob Trousdale
'90 Westy