John,

I had a bad stumbling problem a month or so back.  Started OK, idled
fair when cold, idle fine when warm (idling is another issue), ran
fine about a mile up the road and then stumbled, cut out very badly
between 3000-4000 RPM.

One important symptom was that the tach dropped like a rock, faster
than I believe the engine can spin down.  With list wisdom and
Bentley, I suspect the Hall sender.  The Hall sender is a gizmo that
sits underneath your distributor cap and senses how many RPMs the
engine is doing.  Its signal goes to the tach and the ECU.  No RPMs,
no fuel injection = stumbling.

The wiring harness on mine was cracked badly, so I suspect a short.
After trying to diagnose exactly what was happening, I haven't been
able to make it fail again, even when jiggling the wire.  I
removed the distributor cap, picked out plastic shrapnel around the
wiring harness, replaced the distributor cap and rotor (stumbling
happened _after_ I put a new ones on), The van has run fine since
then.  The real test is tonight when we're driving around 300 miles.

Bentley has a method for diagnosing, but try just jiggling the
wiring harness while the engine is idling.  Also, when driving and
the stumbling occurs, pay attention to the tach.  Does it swing
wildly?

Best of luck,

-Karl

Karl F. Bloss - blosskf@apci.com or bloss@enter.net
'87 Vanagon GL Weekender "Beverley"; Trexlertown, PA
http://www.enter.net/~bloss/vw/