How about pulling off a fuel injector connector. I did this recently on mine and discovered that the problem (exactly as you describe) was an intermittent bad connection inside the connector. Replaced the connector with a GoWesty kit and problem went away, hopefully for a long time. It had been an intermittent problem for at least the last 20 years. No one was able to find it. I was lucky.
 
> Date: Sat, 14 May 2016 05:04:07 -0400
> From: dbeierl@ATTGLOBAL.NET
> Subject: Re: Miss at idle?
> To: vanagon@GERRY.VANAGON.COM
> 
> At 04:49 PM 5/13/2016, Scott wrote:
> >while there, look for an insulated spark-plug wire pulling pliers ...
> >so you can pull off one plug wire at a time while it's idling.
> 
> The problem with doing that is it violates the Bentley instruction
> that it's ok to ground the high voltage, but never let it run open
> circuit (because the resulting high voltage puts too much stress on
> insulating components in the system, possibly including the internal
> structures in the driver transistor**).  And damage of that sort to a
> transistor can be cumulative, so it shows up as a failure down the
> road and not immediately.  This is a hallowed old technique but it's
> not safe with modern ignition systems.
> 
> **Using a scope I've measured roughly 450 volts for a few
> microseconds per firing pulse on the coil negative terminal -- that's
> why the tach bites you if you touch the connection.  If the high
> voltage were not allowed to arc to ground I'd expect the primary
> voltage to go much higher, which would directly stress the extremely
> thin insulating layer inside the driver FET (field-effect transistor).
> 
> Yrs,
> d