In my experience crazy tach behaviors are caused my crazy coil signals. Something is making the signal from the tach act out. 

Oh and how is the fuel flowing out of the tank pre filter?
Meaning could there be anything obstructing the flow at the tank? You know like rust, gum up build up in the tank right at the hose leading from the tank to the pump. I would expect the pump to be noisy with that, but who knows. That is an easy check. 

Sorry to not be of more help

Sent from my iPhone

> On Nov 14, 2013, at 9:20 PM, Gregory Smith <gfs@GREGORYFSMITH.COM> wrote:
> 
> Heeeeeeeeelp!
> 
> I am pulling my hair out trying to solve an ignition miss on my '88 GL. 
> When it happens, the engine is warmed up and I'm usually just cruising
> along.  The engine stutters and the tach dips and even goes up a bit
> sometimes.  The fuel pump is good, quiet, filter is about 2 months old. 
> 
> Initially I suspected it needed and tune up and have replaced the plugs,
> cap, rotor, and wires using the appropriate parts from Bosch.  The
> problem persisted so I replaced the ignition switch with a new one.
> 
> The problem persisted... reading up on the symptoms I thought it might
> be "vanagon syndrome" so I swapped in a different AFM, still there. 
> Added the proper capacitor to the AFM innards, still there.  Unplugged
> the O2 sensor... still there.
> 
> Any ideas?  Does the hall sender in the distributor do this when it's
> failing?  I think it's more of an on/off failure so I'm stumped.  Thanks
> in advance.
> 
> Gregory Smith
> '88 Vanagon GL
> '77 Lancia Scorpion
> Olympia, WA
>