RE: Vanagon Intermittent Syndrome
Just read through the whole thread up until now.
First, Harold Rusts website with his good instructions on the capacitor fix
is long gone.
Second, AFAIK 'Vanagon Syndrome' effects bot the 1.9L and 2.1L engine
control systems.
Third, the factory pigtail works.
List wisdom has it that the problem is caused by the vane of the AFM sitting
in one spot, as on long stretches on the flat at constant speed.  Confusing
signals get sent to the ECU.  So shutting the engine off for even a moment
resets the ECU.  My '90 started doing that to me on a long trip two years
ago.  Dug out the factory pigtail I had in the spares, installed it, problem
went away permanently.
Now to the problem described.  The progressive (getting steadily worse)
symptoms are not quite consistent with Vanagon Syndrome, at least not based
on my experience first in my '85 then in the '90.  Because of the
progressive nature of the problem I'd be looking at some of the other
suggestions along with the AFM problem.
BTW, if needed I might (no promises) be able to come up with a copy of
Harold Rust's instructions on adding a capacitor to the ECU harness.