Kurt-

  John is probably right. It is likely a wiring problem. My beastie had a problem like this once upon a time. The three wires coming out of the distributor were the problem. They had flexed enough to fatigue through. The insulation appeared intact, but the wire inside was broken. The break was about an inch from the connector into the distributor. When the right wire breaks, no power gets in to the Hall sender. No power, no signal. No signal, no ignition.

  To test your van and see if this is the problem, try wiggling that small harness with the van idling in your driveway. Once the wire ends aren't touching any more, the engine dies. And takes the tach with it.

  Fixing this is quite simple. It involves little more than some solder and heat-shrink wrap. My fix has held for 3 years so far.

  <SNIP>
That sounds like a problem with the Hall Sender. It could be as simple
as a wire in the system losing contact momentarily, to the Hall Unit
actually becoming faulty. Cheapest route to start is to check out all
the wiring first, including the quality of grounding points. You may
have some corrosion building somewhere or a wire working loose.